I've had a few dogs in my lifetime and I've had that happen. I know it is heartbreaking! I remember being so afraid that my dogs wouldn't be friends. My dogs always fought once or twice when they were first living together. Once they established who's boss they got along fine. Hopefully that is the case for yours. Now, hopefully, one will let the other eat first, have first dibs on the bones and toys, etc... So there will be no reason to fight. I would remain guarded for a while though. Always be sure to give them an equal share of bones/toys/food and give it to them seperatly.