When he “copies” him, is it mirroring behaviors? Or running along side the fence?
“Mirroring” behaviors is a good sign. This means when one bun grooms, the other grooms, when one eats, the other eats, etc. Could he be trying to groom him through the cage bars? That would also be a good sign.
If they are running along side the fence with each other, this is an aggressive sign. Do either of them mark all over the place when you switch their territories?
Time can heal most wounds, so if they are mirroring each other and not marking all over the place, I would start with pre-bonding for 4 weeks. Swap who is in what cage every day or two.
Then I would opt for a very neutral, large space (2 x-pens linked up works great) for your bonding sessions. The space you use if very important. As neutral as possible, even if it means taking them to a friends house or setting up a pen or two in your backyard.
Start with short sessions with defined time goals (anywhere from 1 – 5 minutes for first sessions), so you aren’t tempted to push it too far and have it turn to a fight.
Since they fought before, when they approach each other, pet them both lots so they think they are being groomed and to keep them calm. Do NOT allow them to fight! Especially early on, since they fought before, make sure they don’t get to that point.
Then you can start slowly increasing the amount of time in the sessions, but always set a time goal, and always end on a good note, even if that’s you petting them until they are calm.
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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.