Mouse bites other mouse?

maybe it a domestic thing. One mouse might be the alpha and want to bite the one because it is bothering her. you could get a stand to hold two 10 gallon tanks so it would be standing up. You could also get a tank separator to put in the middle so she doesn't bite the other one. if you do have a male one could be pregnant and the girls could be fighting over the boy. Or it could be the other way around. you might have two males and they could be fighting over the female