Are both of your rabbits spayed/neutered? If so, for how long? If not both are fixed, that is one large obstacle, as trying to bond while one rabbit is intact means you are fighting the influences of hormones, which is a battle owners can’t really win, since we can do nothing about how their biology will impact them, as well as the behavior of rabbits around them– so, if one is not neutered/spayed, I would halt any attempts and look into doing so. One-month post the operation, you can then try bonding again.

We have a very helpful bonding guide here: https://binkybunny.com/infocategory/bonding/

It does sound like, assuming both are fixed, that some more time in just pre-bonding would be valuable. This means no bathtub dates, but simply living side by side (sharing a wall, if civil; gap if not) and you swapping litterboxes and toys to get them accustomed to each other’s scent. Once you see very consistent non-aggressive behavior/reactions through side-by-side living, that would be a good sign to do daily, short, bath-tub dates. Short can literally mean only 10 minutes a day, and then slowly allowing it to go longer until it can be for hours– at which point, you expand to a bigger neutral territory, and then eventually go to semi-neutral (again, see guide for information; this can typically take weeks).

Re: stress bonding, yes, there are mixed opinions. For some rabbits, it helps, but not for all. Every bonding process is different, and I think of using stress bonding when it seems like going forward has been impossible, despite following recommended practices.

I think what you’re experiencing is pretty normal when bringing a new rabbit home (regarding the aggression at the barrier)! I think Wick hit it on the head with more pre-bonding, I have a few suggestions to add on to that.

When one rabbit is acting really territorial, it can help to set the new rabbit up right in the middle of the original rabbit’s territory. So if possible, I would set up a very secure pen for Rosie right next to Cookie’s pen. If Cookie is free-roam, set him up with a pen too, side by side. Then you can give alternate free-roam time for exercise, or divide the room in half for play time (but you’ll need to use 2 pen walls to create a buffer to protect those noses).

Then I would let Rosie settle into this arrangement for at least a couple weeks with no side swaps, given that you said she was pretty shy so far. With shy bunnies it’s especially important to get them settle in to make sure they aren’t “artificially submissive” in bonding sessions.

Keep in mind, Cookie might be pretty mad about this for a few days, but he should settle down pretty quickly! I’ve had buns running the fence and trying to charge each other one day lounging next to each other a week later.

When Rosie seems like she’s acting confidently, start swapping who’s in which pen every day or two. Do this until Cookie is acting chill with the whole arrangement. Not charging at Rosie or running the fence or just generally being aggressive. Ideally you’ll even see their behaviors sync up (like mutual hay eating, grooming themselves) and they may sleep near each other on opposite sides of the fence.

Once that happens, I think you will be in really good shape to resume bonding sessions and they will both be more calm!