How long have you had the cages side by side? Pre bonding should be done for at least a few weeks, and personally I would opt for at least one month. The idea of pre bonding is that you give each bun sufficient time to become acclimated with the others scent. Hopefully, this would lead to them both being less aggressive when they meet face-to-face.
It may be beneficial to just separate them for a bit, then start up again with the pre bonding. Try switching them into each others cages too so they can use each others litter boxes and such. Every time I switch my buns they have a chinning war lol.
As the others said, if you let each bun in the area individually before you put them in together, it is no longer neutral. Each bun has the idea that this is theirs now, and they will feel the need to defend this space from the intruder bun. That is why neither should be allowed into the bonding space prior to.
I was going to try bonding in my bathtub. Both buns have wandered in there, but neither is allowed in there regularly. But since they are free roam in their own areas, I don’t have a neutral space and I really don’t have room for an xpen. I’m hoping that the bathtub with the shower curtain drawn closed will make it neutral enough.