Do they hurt each other much, or are any injuries trivial? Are they friends again within moments of the fight ending? Some dogs living together seem to think nothing of the occasional short scrap; in fact they seem almost to enjoy it! Some humans living in the same house have loud arguments,even physical fights, particularly if they are related, but are soon friends again afterwards.Dogs are no different.Here we have five bitches who are sisters plus their mother.Our lot get into fights with each other all the time, sometimes one on one, sometimes involving several joining in.There's a lot of noise and the scrap looks dramatic,there are sometimes minor injuries,but within minutes of the fight ending they calm down and are as friendly as before.
You can get the same with dogs who are not related, more,perhaps, with some breeds than others (I defy anyone to keep Hungarian Pulis that don't squabble and scrap!)
The real problem is where you have one dog who is the troublemaker.This may be your problem.That dog will repeatedly either set upon or make to fight the other,'getting the other going', so a fight starts.Commonly the troublemaker has been raised as 'an only dog', for example as the one pet in one household, and never adjusts to being in a house where another dog gets attention.We had a tiny papillon who'd been a lapdog elsewhere.He picked fights, or tried to pick fights, with every other dog we had, sometimes provoking them into exasperated combat.He didn't care what size or how tough they were, they were all his target, and he always lost, but that never stopped him.The only answer was to segregate him: we finally gave him away, for his own safety, because he was heading for being killed one day (and then peace was restored)