Is it possible one of them coul dbe ill? Dogs will sometimes attack and kill sick dogs, so maybe get them both to the vet for a full check up.

What I would say is that you need to be very careful. Fights between bitches can escalate and end in the death of one. I have known people who have had to keep bitches apart their entire lives for this reason, and those who don't listen and ended up with a dead dog.