Are they hens or drakes?
I don't know how long you have had them but keep them separated for at least two weeks to make sure they don't transfer anything. I would put them in "see but don't touch" pens so they can learn of the others existence. I usually let them free range together for a bit...
I've never had a fight to the death but probably would've had one without intervention. That was with roosters though, but i think it would be possible with hens.
Another thing to think of is that sick hens will get beat up on, even to death.