Is there a case of multiple personalities that is not a disorder? Is there a case of multiple personalities that is not a disorder? For some reason everything I found about "multiple personality" comes in a "disorder" form. Or is it a given that one mind can only have one personality?

Is there any information on cases where multiple personalities coexist in the mind of a single individual, without the adverse effects such as disassociation, memory loss or self harm, that is not the product of a trauma?

Note that I do not ask in the context of "social context" personality, but two distinct personalities that persist throughout different social settings. Also it might be worthy to note that even though they are vastly different in the way they perceive and react, there is no pronounced conflict or any adverse side effects as far as I can tell, if anything it appears to have a wide array of benefits.
Well, this is mostly a technicality (but then so is the DSM definition of any mental disorder). The DSM allows for a DDNOS; actually in DSM-5 that would be OSDD "Other Specified Dissociative Disorder" diagnosis:


  This category applies to presentations in which symptoms characteristic of a dissociative
  disorder that cause clinically significant distress or impairment in social, occupational, or
  other important areas of functioning predominate but do not meet the full criteria for any of
  the disorders in the dissociative disorders diagnostic class. The other specified dissociative
  disorder category is used in situations in which the clinician chooses to communicate
  the specific reason that the presentation does not meet the criteria for any specific dissociative
  disorder. This is done by recording “other specified dissociative disorder” followed
  by the specific reason (e.g., “dissociative trance”).
  Examples of presentations that can be specified using the “other specified” designation
  include the following:
  
  
  Chronic and recurrent syndromes of mixed dissociative symptoms: This category
  includes identity disturbance associated with less-than-marked discontinuities in
  sense of self and agency, or alterations of identity or episodes of possession in an individual
  who reports no dissociative amnesia.
  
  
  [more types follow but are not relevant here]


So OSDD type 1 would include any cases of multiple personalities/identities without dissociative amnesia, so in this (DSM) sense you can't have your cake and eat it, i.e. multiple personalities/identities would always be a disorder, albeit with not-so-obvious name, assuming the extent of these personalities/identities is sufficient to be deemed clinically relevant (by the diagnostician). For reference, the brief DID definition (without all the usual caveats of the full def such as not being caused by drugs or other medical conditions, but being clinically relevant in terms of distress/impairment etc.) is


  Dissociative identity disorder is characterized by a) the presence of two or more distinct personality states or an experience of possession and b) recurrent episodes of amnesia.


So clearly if (b) is dropped from DID resulting in OSDD type 1, we're left with what you're asking for (again as long as it's clinically relevant distress/impairment, not caused by drugs or other medical issues etc. These are common verbiage caveats added to most definitions of disordes in the DSM.) So then this is pretty much an issue what is clinically relevant distress/impairment so that a cluster of symptoims is a disorder. There is only one additional, unusual caveat added to DID in DSM-5 (and this would apply to OSDD as well):


  Note: In children, the symptoms are not better explained by imaginary playmates or other fantasy play.


On the topic of prevalence and presentation, a 2014 review notes 


  Chronic complex DD [dissociative disorders] include dissociative identity disorder (DID) and the most common form of dissociative disorder not otherwise specified (DDNOS, type 1), now known as Other Specified Dissociative Disorders (OSDD, type 1).
  
  [...]
  
  The most common type of DDNOS, which has been replaced in the Diagnostic and Statistical Manual of Mental Disorders-5, called other specified dissociative disorder (OSDD), is typically found to be the most prevalent DD in general population and clinical studies with a prevalence rates up to 8.3% in the community reviewed in [...]. Across general population studies, the most severe DD, dissociative identity disorder (DID) has a prevalence of approximately 1% and has been found in .4 – 14% of psychiatric inpatients and outpatients, depending on the sample [...]. Most DDNOS/OSDD patients are similar in presenting symptoms, history, clinical course, and treatment response to DID patients, so DDNOS/OSDD is combined with DID here [...].
  
  Many clinicians and lay people believe that DID presents with dramatic, florid personality states with obvious state transitions (switching). These florid presentations are likely based on media stereotypes, but actually occur in only about 5% of DID patients [...]. The vast majority of DID patients have subtle presentations characterized by a mixture of dissociative and PTSD symptoms embedded with other symptoms such as posttraumatic depression, substance abuse, somatoform symptoms, eating disorders, personality disorders, and self-destructive and impulsive behaviors [...]. A classic presentation includes a history of multiple treatment providers, numerous serious suicide attempts resulting in repeated hospitalizations, and good medication trials typically with limited or no benefit [...].
  
  Although the media and public are often overly fascinated with DID dissociated self-states, the complex symptomatic presentation of DID receives more clinical attention from trained clinicians [...]. Under-recognition of DID is common because the most obvious and pressing aspect of a patient’s clinical presentation may be one of the many comorbid disorders (e.g., severe mood disorders, posttraumatic stress disorder [PTSD], eating disorder, substance abuse, BPD), or the pseudopsychotic symptoms related to the overlap and intrusions of self-states into consciousness. This overlapping influence of self-states causes “passive influence” phenomena or Schneiderian first rank symptoms, which are more common in DID than overt, obvious “switching” of states. DID patients experience more first rank symptoms than do individuals with schizophrenia, with the exception of thought broadcasting or audible thoughts [...]. Intrusions into consciousness may be partially excluded from consciousness (e.g., hearing voices of states, thought insertion/withdrawal, “made” actions/impulses) or fully excluded from consciousness (e.g., time loss, fugues, disremembered behaviors; [...]. 


So yeah, weirdly, the otherwise specified DDs are much more common than the "flagship" DID... and (thus) the transitions between personalities/identities are mostly non-abrupt.

And finally whether DDs can occur without trauma, the DSM-5 says yes, but also acknowledges that DDs commonly occur after trauma:


  The dissociative disorders are frequently found in the aftermath of trauma, and many
  of the symptoms, including embarrassment and confusion about the symptoms or a desire
  to hide them, are influenced by the proximity to trauma. In DSM-5, the dissociative disorders
  are placed next to, but are not part of, the trauma- and stressor-related disorders, reflecting
  the close relationship between these diagnostic classes.


And going back to the review for some empirical quantification:


  Epidemiological studies have found that mood, somatoform, anxiety disorders, and substance abuse are commonly associated with antecedent trauma, as well as PTSD [...]. These disorders are also common co-morbidities of patients with chronic complex DD [...]. For example, in a prospective treatment study of DID and dissociative disorder not otherwise specified (DDNOS) patients, 89% also had PTSD (n = 242), 83% had a mood disorder (n = 226), 50% had an anxiety disorder other than PTSD (n = 136), 30% had an eating disorder (n = 81), 22% had a substance abuse/dependence disorder (n = 61), and 22% had a somatoform disorder (n = 59).


But you have to keep in mind that a study like the last one is restricted to subjects who are seeking treatment, so they experience one or more distressing symptoms.

On the other hand, I'm guessing your question might have been motivated by a Vice article that has a similar title to your question, but which is mainly talking about the "multiplicity community" whose members are appanretly not distessed by their experiences. Likewise there's a "Plural Activism" movement, the mission statement of which is apparently


  Plural Activism will bring public awareness to
  the fact that multiples, no matter their origin,
  are able to live healthy, successful lives
  without integration.


So that goes back to what I was saying about psychiatrists (and their studies) mostly seeing people who are distressed. Quoting from Vice:


  The multiplicity community insists on being seen as healthy—even normal. This is our reality, they argue. Why are you imposing your reality onto us? Dissociative Identity Disorder (DID)—and its controversial precursor, Multiple Personality Disorder—are terms roundly rejected by the community, and most of them don't feel that they belong in the Diagnostic and Statistical Manual (DSM) at all. It's not that they don't believe people can suffer from DID (or, more broadly, Dissociative Disorder Not Otherwise Specified [DDNOS]). They just don't accept that they suffer from it. To them, all those with DID/DDNOS are multiple, but not all multiples are DID/DDNOS. Contrary to what a DID/DDNOS diagnosis implies, multiples want everyone in their system to be seen as people. Not fragments, alters, or personalities, but distinct individuals who happen to be inhabiting the same physical body.
  
  [...]
  
  "The multiplicity community's history with the MPD/DID/DDNOS labels is complicated, and full of contention," says Falah Liang [an exponent of this community]. "There's a lot of resentment towards psychiatry for pathologizing what is seen as simply a neurological difference like being left-handed, for painting multiplicity as freaks and invalids, and for pushing integration as a necessary 'cure' that all multiplicity must undergo." On the flip side, the anonymous psychologist I spoke to expressed mild resentment at the idea of healthy/empowered multiplicity distracting from people who actually need a DID diagnosis. "People with DID need to be recognized, as it's a genuine disorder," she said.
  
  [...]
  
  Dr. Max Krucoff, a neurosurgery resident at Duke University Medical Center, would be the first to acknowledge the potential slipperiness of DSM diagnoses. "Most medical diagnoses are defined by a pathophysiology—something identifiable, anatomic," he says. "Psychiatric diagnoses have no known underlying cause. I can't take a biopsy of somebody's brain and look at it under a microscope and say, 'You have Multiple Personality Disorder.'"
  
  Still, the controversial term "disorder," he says, is a means to an end: effective treatment for those who want and/or need it. "If people aren't hurting anybody or dangerous to anybody or asking for help, there's no reason to go looking for them and treating them," he says. "Everybody's on a spectrum, and what's considered a psychiatric illness is often evolving. If you feel like you're sick, if you feel like you need help, then doctors may be able to label you with a disorder only so they can figure out a good way to treat and help you."


And I was going to say that no academic publications have looked at this multiplicity community, but I'm glad to be proven wrong, there's a 2017 paper: "Multiplicity: An Explorative Interview Study on Personal Experiences of People with Multiple Selves" mostly exploring the on-line communities:


  Results: Multiplicity is discussed on Twitter, Tumblr, Google+ and several other personal websites, blogs, and forums maintained by multiples. According to the study's estimates, there are 200–300 individuals who participate in these forums and believe they are multiple. Based on the six interviews, it appears that multiples have several selves who are relatively independent of each other and constitute the personality's system. Each “resident person” or self, has their own unique behavioral pattern, which is triggered by different situations. However, multiples are a heterogeneous group in terms of their system organization, memory functions, and control over switching between selves.
  
  Conclusions: Multiplicity can be placed along a continuum between identity disturbance and dissociative identity disorder (DID), although most systems function relatively well in everyday life. Further research is needed to explore this phenomenon, especially in terms of the extent to which multiplicity can be regarded as a healthy way of coping.


And a few more details relating to antecedent trauma (or lack thereof), 


  Typically, multiples use their unique terminology to refer to common experiences, for example, “system” (which is the term used to refer to themselves, i.e., a system of persons), “resident persons” (or “alters,” who are alternative personalities sharing the body), “fronting” (when one resident person takes control over the behavior in a particular moment or period of time) and “host” (the original personality, often the one who has been present from birth). [...]
  
  Most systems do not report amnesic barriers or recall traumatic events, and they insist that their multiplicity is something they were born with. Many of them call this healthy or a natural state of identity.
  
  Multiples are a heterogeneous group. According to their posts, their triggers, how they switch between resident persons and the way they organize real-life behaviors differ greatly from one system to another.
  
  A recurring topic on forums is the gender identity of multiples. Many multiples experience transgender issues and gender dysphoria because their residents have different genders and sexual preferences. There are many similarities between multiplicity and lesbian, gay, bisexual, transgender, and queer (LGBTQ) rights activism. Multiplicity uses many of the terms that the LGBTQ community uses, such as “coming out,” which means “revealing themselves” to the outside world as a multiple.


The first 3 vignettes (interviews) in the paper do involve antecedent trauma (two of them quite clearly) or report switching being somewhat stress-related. The 4th vignette is perhaps an example where trauma is not involved, but it does involve autism. The 5th vignette is perhaps the one with the least co-morbidity:


  “I'm the host. I'm agender, because I don't like to be identified by gender. We've decided, it's better that I talk because I've been here the whole time,” a 22-year-old female voice introduced herself. She lives in a small town with family and currently studies psychology in college. [...]
  
  The system started about seven years ago, when the body was 15 years old. “We are not traumagenic; it just happened,” she stated. “In the very beginning, there were only two people who argued. One day I woke up and felt as if someone was possessing me. First I thought that I was losing my mind. It was crazy.” Then, a couple of months later, another person switched in, but it did not communicate with her. A little later, someone else came in and established order. Then, more switched in. There are fairly original members, and there are other ones who gradually showed up.
  
  Her parents know about the system, but they have never met other members. “My mum asked me if I wanted to kill her,” she stated. Her teachers do not know about her system because she would never allow switching in the classroom. She talked to professionals, and they do not think it is a problem; they see it as a sort of help. Given that she has not had trauma or amnesia, they do not believe it is DID. She added, “It isn't interfering with my life now. On the contrary, it is helpful.”