If they don't get along, I wouldn't make them share a cage. That would be like sharing a house with someone you don't get on with. It wouldn't be fair on either one. They probably won't try to breed (unless they got really attached to each other and had the right breeding conditions).
Also, just because they're similar birds, doesn't mean they'll get along. I had three galahs together in an aviary (two males, one female). The males would always pick on the female, so I had to separate her. Even though they're the same species, they don't always get along. Also, they never tried to breed. Just because a male and a female bid are together, doesn't mean they'll breed.