I am amazed this is such a major issue with some people. I have until recently been working for an organisation with in excess of 150 pilots and we regularly flew two captains together. As far as I am aware it was never raised as a problem.
Yes, I would agree that there is the potential for CRM issues but that was dealt with as part of routine CRM training. Base checks for captains were done in alternate seats each time so we were fully assessed for handling competence in each seat once a year.
I would agree that for cost reasons this is not ideal but if you have a 50/50 split, captains verses non captains, one could have a situation with a higher number of captains sick or unavailable for a period and you then end up with co-pilots as useful as the pope's testicles. We used to have about a 60/40 split and it used to work fine.