Is it healthy for a child to be raised by two males or two females? How would they distinguish the role of mother and father?

Study after study shows that they're just as healthy as other children. The actual relationship between the parents, and between parents and child have a far, far larger effect than anything to do with gender roles. Gender roles are largely societal anyways, and there is a HUGE amount of variation even in more "traditional" families.

I think it was XKCD that said something along the lines of "isn't it funny how evolutionary psychology always seems to confirm stereotypical 1950's gender norms?"

Yes its healthy. There is no reason the performance of the roles has to be based just on gender, if they were then single parenting would not function either.

"How would they distinguish the role of mother and father?"

What are these roles? How do children in heterosexual families "distinguish" them? What does "distinguishing" them mean to you? Why is it important that children learn to distinguish them? What happens if they don't?

Up to a certain age of say 5 years, the demands of the growing kid & the way thy can be best addressed can be ably be done by females rather than by males. As the kids grow beyond that age, the kid will have to be shown the exterior world, face & understand it piece by piece can be best be done by males.