The nipping is part of the process and as long as they are not nipping too hard or chasing each other I think it would be okay to let them continue deciding who is boss. When they nip, try pushing their heads down cheek to cheek, say 'be nice'. Hold them there until they seem to relax and then let go. I'm sure you can tell though when to end the session as you don't want either of them to be stressed out.
I can't tell which one of my buns would be the dominant one though. Lucky was more aggressive with nipping and thumping but Molly did the mounting and always has her head down wanting to be groomed...and Lucky grooms her now.