Is there a conflict of interest when the supervisor and co-supervisor are husband and wife?
Your supervisors are problematic in several ways, from what you describe:


Pushing their own research interests over what's most beneficial to you
Exposing you to unpleasant interpersonal issues between themselves that affect your working environment
Not coordinating and communicating effectively with each other to co-supervise you


However, none of these things are necessarily the result of them being married. The same issues can arise (and I have seen them) with unrelated co-supervisors who do not get along.

In general, co-supervision by a married couple is not universally disallowed.  (In my university it is allowed, and there is a husband-wife pair in my department that successfully co-supervises PhD students on occasion.)

Some universities do recognize this as a conflict of interest. This from the University of Western Australia (emphasis mine):


  Supervision must be free of actual or perceived conflicts of interest.  Supervision by, or co-supervision with, close relatives or those in close personal relationships is generally not appropriate.  All such cases must be declared at the time that supervisors are nominated.  In cases where approval is given on academic grounds for supervision by, or co-supervision with, close relatives or those in close personal relationships, additional supervisors who are free of an actual or perceived conflict of interest must be appointed.