I'm not sure how it is with mice, but I know rats play fight a lot (especially the boys) and sometimes tackle and beat up on each other to show dominance. The submissive one will usually squeak to show that he is "submitting" to the boss. I assume it is similar for mice, but again, I don't know much about them. As long as there is no bloodshed and the submissive guy isn't being injured or excessively punched around, I wouldn't worry too much about it. Keep an eye on them...