Make being together more trouble for them than being apart. If you are near the other horse, tinker with the one you are on. Make him work, etc. When you ride away, let him have a break. Same deal about the barn, tie him short in his stall with just a water bucket, don't feed him in the barn...make the barn a place he does not want to be.
You will see a difference. May never be the horse you want, but he will improve over time.
X2