My girls do this too, but it's getting better. Remember to have a spray bottle set on stream with you.. good for ending tussles, then they will start to groom. I've heard that grooming is an accepting action, so grooming is good! When my girls tussle, I spray them to end it, say a stern NO and then put them side by side and cuddle both and sometimes give them some oats. My girls seem to do this when one of them nips, then the other gets upset and bats, then they start tussling. Remember to check both after to make sure there's no bites or bad scratches if it's a bad fight.
You could try shorter bonding sessions, and as they start getting better and better with each other, increase the time. I found I started with sessions too long (45 minutes- 1hr) in the beginning, and that's when I got the tussling. Now that I've been working on 20-30 minute bonding sessions, I rarely have a problem.
Hope this helps! Hopefully Buddy and Roxie's bonding sessionswill improve!