Why did you think they were both girls? Did you come to that conclusion on your own or did someone where you bought them from tell you? In either case, it is important that you seperate them if you still aren't sure, because even if they have mated, they will likely continue mating, whether one is already pregnant or not. And if she's not, then you can ensure she won't be by giving them seperate homes until you can know their sexes for sure.
If mice are old enough, it is pretty obvious, because males have testicles that are very easily seen (LOL, there is no pretty way to say that
). However, if they are young enough it is harder to tell. What I suggest you do is look under their tails...in a female, the anus and the little flap you will see further towards her belly are almost touching, while in a male, the space between is significantly larger, as this is where his testicles are (even if not visible yet).
Hope that helps, and good luck! To ensure the health and happiness of your mice, it is best to buy same-sex pairs (just to let you, and anyone else, know in the future). Where babies are due, there will no doubt be costly expenses, trips (possibly) to a vet, and even a mother who cannot handle labor.
I hope everything goes smooth with you, though! There is a chance that they really ARE two females just trying to win dominance over each other, or, sometimes, mine will sniff eachother's rears and act funny when they are in heat!
Another important question you may have to consider:
What would you do if you DID have mouse babies?
Keep us posted!