Ahhh... this is a tough one. We've had this experience before and I don't have the proper personality to be able to overcome this problem. My husband does, though and he was able to keep control of his 2 unaltered male dobermans. It's more than likely that the 2 females are vying for the top dog spot or fighting over a resource. They can be buddies one moment, and a trigger presents itself and the fight breaks out in 3 seconds flat.
The ideal scenario is that one dog defers to the other dog so that all the top dog has to do is issue a warning and the other dog defers. If this does not occur naturally (if you have 2 wanna-be-queen dogs), then you have a problem. The solution to this is to let both dogs know YOU are the top dog and that behavior is not acceptable... so that YOU are the one to issue the warning and both dogs defer. I couldn't do this because I can't seem to establish myself as top dog - might be a personality trait. My husband has a natural talent to be top dog - he's never gone to any training or anything yet he has this way of getting the dogs to fall in line. It takes a while for them to learn and accept their place, so you may have some tough days ahead. With our 2 dobermans, we spent $800 to get both dogs stitched up before they learned to quit it.