Aww those type of pairings can be very frustrating, but the good news is it’s better than 2 aggressive buns! What’s the timeline here? How long have you had Truffle? And just confirming both are spayed/neutered for at least 3 weeks? It’s important to remember that aggression is based out of fear. Even though Po has been passive, Truffle is in a new place so it’s normal for her to feel some fear.
I think my top recommendation is to switch up the housing. You made the right call by moving Truffle’s pen into Po’s room. I would go one step further and get a second pen and house Po right next to Truffle with a nip-proof barrier (either a buffer of 6 inches or small mesh panels). I’m not 100% sure of the timeline here, but I would allow them to get used to this set-up for at least a few days or a week. Then start swapping which bun is in which pen each day or two. Let each bun have free-roam time, I like to let one bun have free roam in the morning and the other in the evening. Again you have to make sure the fencing is nip-proof, so you can add an x-pen to create a buffer or use small mesh panels.
I recommend you pause face to face dates and keep up the side-swapping until Truffle is no longer reacting aggressively towards Po while they are in their pens. If you notice she is REALLY freaking out about this arrangement, you can add a towel or sheet as a visual barrier between them and then slowly remove it. That will help her get used to his scents first. You can also pet them both and swap scents. Some people like to add a stuffy to each pen as well for that reason.
Once both buns are relaxed with the side swapping you can resume the dates in neutral space. I think a larger space would be better for this pair, so Truffle won’t feel as threatened by Po moving a round slightly, as she will have more space to move (and he will be able to move away from her if needed).
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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.