Horses don't hate each other. Well trained horses just don't attack other horses through their stalls. I've had horses in small herds for over 27 years, and the fights are always over food. ONLY poorly trained horses, like my first herd leader, behave this way, and these malcontents will kick the stall wall no matter which horse in next door.
Horses will jostle for position and they always want to know who is the boss. THAT is why the owners have trouble with these two, bc they haven't established their leadership. At some point, the owners of these two horses are going to get hurt. If the OP moves and gives a report, then I'll post again. For now, I'm done with this discussion.