I have played around with the angle of my screen and IF what I think I am seeing is what it is, then Stympy is a also a girl. This would make more sense to me with 10-11 months old degus which haven't had babies or a serious fight so far.
I'll probably get shot down for this but yes that's what i'd do. Providing there are no serious injuries or they are fighting in silence, which would indicate it was serious then they need to establish a top goo in order to live together peacefully. From what you say Stimpy is the less dominant of the two, so I don't think it will take too long for them to sort it out.