Hi there, welcome
And thank you for adopting!
I think it still might be possible, but I would try a few different techniques.
First, I think some addition pre-bonding might help, especially since the first round occurred when Bella might have still be hormonal. I like to completely swap who is in which cage daily or every other day if it’s possible. Anywhere from 1 week to over a month, depending on how the buns behave. You will likely see an increase in marking and loss of litter box habits at first, but over some time they will calm down. You are basically looking for them to be relaxed near each other, without aggressive behaviors through the fence (attempting to box, running the fence, etc.). It sounds like they have already calmed down a bit on that front, so that’s good.
When you are ready to start sessions again, since your initial first date in the bathroom did not go well at all, I would opt to try a stress session right off the bat, like a car ride. My first pair (the ones in my avatar) fought on sight at first, and the only thing that got us past it was car rides. You’ll want to have a helper drive. You will site in the back seat, with oven mits or tennis shoes or work gloves on your hands. Have the car running, pop the bunnies in a plastic tub, and take them for a drive. Make sure they do not fight! You can pet them both and swap scents. It helps to have them site by side, heads facing the same direction, so they don’t start circling. I would go for about 10-15 minute drive if they are doing OK. Then put them straight back in their areas. Repeat the stress sessions daily for a few days in a row, and see if they seem to be going to each other for comfort. Of course make sure that they aren’t getting too stressed. Some bus can’t handle this tactic, so you might need to reassess.
Once you are ready for longer sessions, I would opt for as large a space as possible, in as neutral a space as you can find. A couple of pens set up in a garage or at a friend’s house can work great. Start them on opposite sides, and hopefully they will spend the first few minutes exploring. When they come together, be ready to pet them a LOT to calm them down. Keep petting and swap scents. Do NOT let them fight. Start of with short time goals (30 sec, 1 min, 3 min, etc), and always end on a positive note, even if it’s you petting them both.
Hopefully with Bella more calm things will go more smoothly for you! It can take several months for female hormones to settle post-spay, so that very well could have been affecting things before.
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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.