I have two bunnies named Mr. Bunny and Princess Potato. Mr. Bunny, our first bunny, is territorial and took some time to warm up. The shelter informed us he is a hard bonder, but he has become very social and even allows strangers to pet him. Since we are often away, we thought he might need a companion. Six months later, we adopted Princess Potato, who is his complete opposite and loves attention.
We gradually introduced them, using stress bonding techniques in a cardboard box and on a table. Eventually, they became so close that I worried they might accidentally bite each other. We then moved them to a smaller pen, no larger than 2×2, and they have been fine. They occasionally nip at each other to assert dominance, and Princess Potato enjoys grooming Mr. Bunny, though he rarely reciprocates with a kiss unless I put treats on her head. They nuzzle each other voluntarily, and there are occasional mounting episodes, about one or two times per hour. After I separate them, they are fine, but Mr. Bunny persistently tries to mount her head, which is not acceptable. I also rotate their pens and toys daily, and their cages are adjacent, which they tolerate well, even overnight.
However, when I place them in a larger shared space with a litter box and hutch, such as moving Mr. Bunny into her primary area or into a cage with only their shared blanket, Mr. Bunny becomes aggressive, nipping and trying to start a fight. What should I do?