Hi there, I recommend reading up a bit more on the bonding process. Here is a good starting point:
https://binkybunny.com/infocategory/bonding/
First you need to let the new bunny settle in for about a week or two without interacting with the other bun. You should have their pens set up side by side so they can start to get to know each other. Next you should swap which bun is in which pen every day or two for another week or two, to help them get acclimated to each other’s scents and loose track of territory. If one bun is fully free roam then it would be helpful to pen him as well temporarily. He won’t like it, but it will be helpful for the process.
Next, while continuing the cage swaps, you will do supervised dates in a neutral territory, so this should be an area of your home your original doesn’t go. This can be tricky with a free roam rabbit, but bathrooms are often good candidates.
In bonding sessions, your goal is to prevent fighting. Mounting is OK, but if the bun being mounted tries to escape, prevent the mounter from chasing for more than a couple seconds. Tight circling should be stopped right away as that often escalates to fighting. Reverse mounting should also be stopped, I usually spin the mounter around so they are in the correct position. If they come head to head, pet them both a lot to calm them down, this mimics grooming behavior and helps build positive feelings between them.
Start with sessions 5-10 min long and work up to several hours each day. Eventually you will work up to all day sessions, then overnight. Your goal is to get them to spend 48 hours together straight without any fighting or aggression and with lots of positive behaviors, like cuddling and grooming. All of this should be in neutral territory, then you can work to transition them to semi-neutral and non-neutral space.
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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.