I’m so sorry you haven been going through this, very stressful!
I suspect that Leo’s illness may have been what initially caused the fight, but it sounds like this might have been a pretty tough bond to begin with. There is a possibility that they just aren’t a good match and/or are holding a grudge over the fight. I think in your case it would be good to take a longer break with no contact between them (ideally out of sight and smell range) for at least a month (maybe 2). This will give everyone a break (including you) and let them forget each other a bit. Also be extra sure that both buns’ health is good and there aren’t any underlying issues that could affect things.
Then go back to your pre-bonding (living side by side with side swaps every two days) for at least a few weeks. Try to build up positive associations during this time, so feed them near each other, etc. Once you start bonding sessions again, I would opt for a very large, very neutral, and even slightly stressful location. For example, 2 x-pens linked up in a friend’s house or your backyard (assuming they don’t go outside). I have found large spaces work best for tough bonds as they have space to move away from each other without things escalating to a full on fight.
I’m sorry there isn’t an easy solution! If you opt to stop trying to bond them, they could very well live happily as neighbors. Even when not bonded, buns benefit from having another bunny around, and there are stories of “failed bonds” that after a year of living as neighbors “suddenly” decided they wanted to be friends.
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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.