Separate them, for now. They’re too young to be mated, and they might be hurt. When they’re older, maybe you can try again.
It's normal for new ducks to have squabbles and pecking order fights at first, but right now I think they're too young to be introduced. My ducklings get to stay with the flock whenever mama lets them, and I've never had any issues with extensive fighting (except for the occasional peck when a baby tries to rudely bust past an older duck). The males never try to mate them until they're at least four months old. I figure their mom teaches them and keeps them safe - but you don't have a mother duck, so it's a different case.