First I would check the harness and the dynamics between the two when trotting, since you say that is when the biting happens. Biter is unhappy about something! Maybe his buddy is pulling him along from behind? Or creating some other uncomfortable situation. Are you using an evener? It may be banging the slow one in the hocks. Can you get both in draft and on the bit? Dose the biting stop then? Try circling left & right and see if the biting stops in either direction. If the biting stops when the biter is the inside horse (and doing the work (and bending correctly?)) then try keeping him in draft and doing more work. Or you can use a Jockey stick! You can also rig a side rein on the outside to limit his ability to reach his partner, but this has other consequences and is not a good long term solution.