If they do well in the tub, I would keep going with that for longer and increase the length of the sessions. 3 weeks is not much time at all. You could try doing a 2 hour session and see how they do.
Sticking to opposite corners in a larger space isn’t necessarily a bad thing. It still means they are choosing not to fight, and they will start interacting more when they are ready.
It doesn’t sound like you necessarily need to do any stressing, but if you do, have one person drive, and the other person sits with the bunnies and makes sure they don’t fight (have thick gloves on). Have the car running and ready to go. I usually bring a spare carrier in case I need to separate. If they aren’t fighting in the tub, they prob won’t fight in the car.
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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.