Some mounting is normal, and if it doesn’t lead to fighting that’s a good thing. They both will probably show some dominance displays to try to sort out who will be top bun. Preventing fighting is the top priority, and I’ve found that in small spaces things can escalate to fighting quickly because they don’t have space to get away from each other.
In the early phases, don’t be afraid to have your hands on them. I know this differs from some advice you might read online, but I have had a lot of success with this approach (I’ve bonded multiple pairs with this method, I learned it from the coordinator at my town’s rescue). When the buns approach each other, I like to pet them both to make them think they are getting groomed, which helps everyone stay calm and builds a positive association. You can also do this if they get too nippy. I will even position them side by side when in a small space like that and pet them both to build some calm feelings. If one goes to mount and the other tries to get away, you can pin the mounting rabbit down to prevent them from chasing. In general, light nipping and mounting should be allowed, but face mounting, hard biting, boxing, circling, prolonged chasing (>3-4 seconds) should be stopped. If they try to face mount you can kind of spin the mounting bun into the correct position.
Once you get more of a feel for how things go with them, you can ease off how much you pet etc., but in the early stages it can be hard to predict behaviors and known when to intervene (esp for first-time bonders!).
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.