That all sounds super normal, don’t worry! It’s very normal for rabbits to react this way when another rabbit is in their home turf (imagine a stranger just set up shop in your house, you might be alarmed as well!). I would let Tess settle in for a bit, just living side by side as a neighbor for Gideon for at least a week. Then after that you can start swapping who’s in which cage every day or two. You might see Gideon acting crazy for a bit but he will calm down. I’ve often seen rabbits that are running the fence and trying to “get at” each other one day, hanging out peacefully on opposite sides of the fence after just a few days.
You’ll want to keep up the side swapping “known as pre-bonding” until they seem very chill with each other. The amount of time this can take varies, but 2 weeks is probably avg. Pre-bonding can be especially helpful in cases of tougher bonds.
Gideon also wasn’t neutered that long ago, so his hormones are still settling a bit. If Tess is from a rescue, she might have been spayed recently given her age. So a longer pre-bonding will also help ensure everyone’s hormones are settled.
The advice you’ve read about the buns ignoring each other is referring to when the buns are in a completely neutral space, such as on a speed date. That doesn’t apply when the buns see each other in non-neutral space.
One thing you can do if Gideon really seems overly stressed is to hang a towel up between their pens for now. Then they can get used to scents first, and then you can gradually remove the towel.
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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.