Yes I’ve often heard (and it was so in my case too) that the subordinate horse at home will become the biter when they’re working. It’s like they realize this is their only chance to get a shot in at the boss, because in harness they can’t retaliate.
I put an outside rein back to the girth on the offender, and also a flash noseband to keep his mouth shut. And of course a scolding when he tried it. It didn’t last long and we were able to remove those measures.
Rachel