Do many people have an "Accompanying Voice" - an equal they converse with?
From a quick search online I see no reason why you distantiate what you describe from auditory hallucinations:


  a form of hallucination that involves perceiving sounds without
  auditory stimulus.


Given the article you link to, which arguably does address your question (yes, some people report experiencing this), you mainly seem to be concerned that this would unequivocally be considered a mental illness. Your actual question thus seems to be "Is hearing multiple voices always considered a mental illness?".

Judging from the Wikipedia article (and its subheading 'Non-disease associated causes') this is seemingly not the case.


  However, individuals may hear voices without suffering from
  diagnosable mental illness. (Thompson, 2006)


The article by Thompson mentions a study for which unfortunately no source is provided, thus I would take the following with a grain of salt:


  But studies by Dutch researchers that began in the 1990s found that
  some healthy people also regularly hear voices. ... Many of the people
  who contacted them did not find the voices disruptive and had never
  felt the need to consult mental health services. ... The resulting
  studies found that more people might hear voices than psychologists
  had thought, perhaps around 4 percent of the population.


It might, however, set you on the right path to finding the actual studies, or more recent work in regards to how many people report hearing voices not attributed to the self without considering them a mental illness.


Thompson, Andrea (September 15, 2006). "Hearing Voices: Some People Like It". LiveScience.com.