Oh man, how scary! Especially with one bun so young! Have you taken your young bun to the vet to have his lip looked at? This type of injury happened on one of my buns and she needed 1 suture to stitch it up.
So, as you probably are aware, it’s essential that they are not able to escape and get to each other again. It’s very normal for rabbits to be aggressive to other rabbits when they are not in neutral territory, especially females. That said, sometimes bad injuries can prevent a bond from forming correctly.
In this case, I would recommend full separation of the buns (so they are in different rooms) for at least 4-6 weeks. You also will need to get your new bun neutered. After he is neutered, his hormones have settled for at least 3 weeks, and that 4-6 separation period is up, you can set them up in side-by-side pens, but with a barrier that they cannot nip through (either fine mesh or a double walled fence with a 4-6 inch space between the walls) or jump over. Rabbits will jump much higher than you’ve ever seen before when they are trying to “get at” another rabbit.
This side-by-side arrangement is called pre-bonding, and you should have them live like this for a couple of weeks and see how they react to each other. Swap which bun is on which side every day or two, and continue this until the buns seem calm around each other (still on opposite sides).
If all of that goes well, then you can try dating in neutral territory (somewhere neither rabbit as been) and see how it goes.
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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.