Generally when cats live together, they learn to get along or at least ignore each other to avoid constant fighting. The big problem occurs when someone brings new cats into a house of existing cats.
Because cats are territorial, they’ll often fight each other. The simplest solution is to just keep fighting cats separate. That way they can’t get into any fight if they can’t get to each other in the first place.
To read advice for dealign with cats that visit a house that already has cats, click here.