Hi, did you ever do prebonding? That is the first step in bonding.
I do have to say, the second bonding way you mentioned, is a really easy way for a bun to kill the other or make them hold grudges that prevents bonding.

So the buns should each have their own pens they are in, dont let one sleep with you. The pens should be separated by 6 inches so neither one can bite. Having a bun free roam means they can go up to the pen and terrorize the other one when they want. Then you switch each one everyday for a month or so, prebonding take a long time to do but it ensures a right bond. Then after a month or so after they don’t show aggression in the pen they are in, you can do bonding dates in neutral territory.

Actually keeping them in pens doesn’t cause aggression, it actually helps each bun get confused with what pen is theirs since the others scent is in it which then leads to them not having their each territory so they won’t have anything to fight over when you do bonding dates. The whole idea of prebonding is to make each bun loose track of who’s pen is who’s so they don’t have anything to fight over.

It sounds like the ‘german method’ is basically marathon bonding which is how I bonded my two. It’s definitely a technique that can work HOWEVER you need to be sensible about it. I pre bonded mine for over a month (I think 6 or 7 weeks) and then put them together in neutral for 4 hours. They booped noses immediately and Pea groomed Buttercup. I separated after 4 hours simply because I was tired. I put them together the next day and they have been together since. I supervised 24/7 (me and my partner took shifts) for 2 weeks straight. Literally. I think by day 10 they had established the hierarchy but we waited a full 14 before we declared them bonded. It’s a very intense method. It’s important to note that this method wouldn’t be appropriate if the rabbits were aggressive towards one another, if one or both were really scared, if they were so nervous that they stopped eating etc.

The difference I see between the german method and the marathon technique is: pre bonding. Pre bonding is essential, so whilst I think there is some good behind the concept of the german method, I don’t agree with the no pre bonding aspect. Pre bonding helps each bunny become accustomed to each other’s smell and helps them almost lose track of what is ‘their territory’ I will also stress that pre bonding is something that should be done for at least a month, not just a few days.

I also want to stress that they must absolutely not be able to touch through bars. It sounds like they were able too? You definitely need to stop that because if they can bite through bars, they will if they want too and that itself can cause issues if they hold grudges (and of course they could end up injuring one another)

I think if I were you, I would stop sessions altogether right now and pre bond for a month. Ensure that there are additional barriers so that neither bun can bite or pull fur and even think about adding a stuffy so each bun can take out any aggression on that.

I can understand in a way the idea that pre bonding can cause pent up aggression but that’s why we say pre bond for at least a month so that they, well, get over it basically lol it allows them time to come round to the idea of sharing a space and so hopefully by the time they have an actual physical introduction, there is no aggression left. The stuffy can be good because it allows them to get their aggression out in a safe way. If the rabbits are aggressive towards the stuffy at first but then after a few weeks, it no longer bothers them, this is a good indication that they aren’t so angry anymore and you can begin thinking about an introduction.

It’s impossible to say at this time whether or not the marathon technique would work for you because you need to do pre bonding first and assess how each bun is after a month of this but it’s certainly possible but as I said, you have to be sensible about it.