Did the dogs behave this way when you took them in? Did the first fight occur over food or bone or toy?
I agree with Dog Dancer that you begin or continue the training. I would keep them separate when you cannot monitor their behaviour and be super vigilante when they are together i.e. immediately correct a "stare down" etc before things escalate. IMO, neutering both dogs is a good idea as intact males need to mate at least twice per season. If they do not get to mate they will have a need to release that energy and it usually results in a fight.
There are many more benefits to neutering which I'm sure your vet will discuss with you.