It sounds as if it is going well so far. A good combo with Asa being such a love-bug. Just doing more of these sessions will go toward the trust building. You will hopefully see Kadeja becoming more relaxed about Asa moving around. You could try setting up a neutral pen if the bathtub is getting tiresome. Experiment with covering the sides with a sheet v’s leaving them uncovered.

I personally would try discourage the belly nips if I can see them about to happen.. Sometimes you only need say “uh-uh” or make a short, sharp noise to prevent it. Sometimes you need reach in and steer their head away (be careful she doesnt turn on you).

I recently have put 2 of my single buns together for a trial meeting. The female does this too. The male was just curious about the space and moving about, and she would sometime put her head down and try go for his belly or butt. She was bonded to another male for 4 1/2 years until he passed away in September. She did do this a little in the last year or so but he was not bothered. It wasn’t actual biting, just pulling tuft of fur occasionally but usually not even that.

With a new relationship, it can be more problematic if the other rabbit reacts badly. I think it’s Kadeja probably testing the waters a bit, somewhat dominant.You might see her do it, then pop her head under Asa’s head to try get grooms also.

It could also be learned behaviour from her past. (My female was kept in a group of rabbits and had one litter that I know of, possibly more) The other rabbit taken with her was young and had a bad bite injury on his back. We were told it was her son. We don’t know if she did it or another rabbit in the group).