The biting at the fence is normal at first, just be sure they cannot nip each other through the bars (so have a buffer of about 4-6 inches, or use small mesh that they can’t nip through. Aggressive or territorial behaviors during pre-bonding include running the fence, grunting, trying to box through the bars, etc. You will be surprised at how high your buns can jump and climb during pre-bonding, so be very sure they cannot get to each other during this phase. Also don’t be too alarmed if you see these behaviors, they should settle down in time. But it sounds like yours are pretty chill overall, so that’s good.

You might notice when you first start swapping cages, the rabbits will run around a lot, chin everything, mark all over outside the litter box, etc. They might run the fence and try to “get at” the other bunny. Then after a while, they will barely seem to notice they are in a new territory. They will not mark as much, they won’t run the fence, they might even lay near the other rabbit on the opposite side. You also will notice their behaviors start to sync up (they will groom at the same time, eat hay at the same time, etc.). Once you see a clear shift from territorial craziness to relaxed behaviors, you are usually good to start sessions. Never hurts to add in a bit more pre-bonding though if you aren’t sure.

Ok great

One more thought, even if you are supervising, only let them come into contact with each other in neutral space during a bonding session.