My two puppies keep fighting?

I have an almost 7 month old shepherd mix and a 6 1/2 month old golden mix and they keep fighting. I know that you will say that that they are just playing, but sometimes it gets really out of hand, like one gets under the coffee table and the other tries to follow her and they bight and snarl at each other, and are constantly showing their teeth, and then when I try to break them up, one of them nips at me. I don't know if it has anything to do with them being separated all day, because one goes in the backyard and one we have to tie on a long leash in the house because she just got fixed. Can anyone tell me how to prevent or at least lessen the fighting?