Here’s my thoughts, but they are solely based on my experiences and what I’ve read.
It might make sense to take a small break (maybe a week or two) and give your buns a chance to relax for a bit. Put them out of sight from each other. Then go back to square one with stress bonding.
Were they fighting in the bathtub? While we were bonding, if Pumpkin started humping and Rocky started chasing, we would pull them together face to face and smoooosh their faces together and pet them while they got calm. Sometimes they would continue to lay like that for a few minutes after and we would end the session that way. Always try to end on a good note.
I’m sure you’ve checked out the bonding tab on here (which has excellent tips!) and you should also check out http://www.thebunnychick.com. It’s ran by member tanlover who has successfully bonded two sets of four! She has some great pointers as well!
Remember if you are stressed they will sense that. It’s ok for you to take a break too!