Anywhere they don’t normally spend time is neutral, so that could be an area of your home that’s away from their areas (such as a bathroom). I think given their history, doing bonding gradually may be the safest bet, so you can sure their bond is very strong and won’t break again. So you could either bring them to the space you are currently using for bonding sessions, or use a space within your house (or backyard, or a nearby friend’s house). It also may help to house them side by side, and swap sides every day or two, rather than in separate rooms, if possible. Ideally you want to see them basically ignoring the other bun through the barrier. If they are showing aggression through the barrier I would wait to do bonding sessions until they calm down.
One thing you might try, that I was discussing with someone in another bonding thread, is doing some side-by-side training with them during bonding sessions. I was having a discussion with a friend at a rescue I used to volunteer at about the trio I’m trying to bond. She said when they introduce new animals to a group at zoos they will often do training with the animals so they focus on something other than the other animal. They start with them farther apart from one another and then have them move gradually closer together. If you work with your buns a bit to clicker train them this could be something to incoporate.
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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.