My two do this. The one going doesn't seem to mind as much as the one left behind. The other day I was advised that I should separate them where they couldn't see each other for 30 days, with no contact at all, and that would fix the problem. I chalked that up to very bad advice & dismissed it for several reasons, the biggest one being that my horse gets so upset when left behind; he runs the fence, trips, falls, slides, etc. and screams till the other returns. He shoots diarrhea down his leg. So I see no reason to stress him out like that, or possibly have him injure himself. Mine simply loves to be around other horses. My husband and I were riding and his horse was acting up as usual...see my other threads...when we happened upon a neighbor riding alone who invited us to go along. My guy was like "Sure! Heck with you, stable mate!" and off we went for the most pleasant ride I have had yet, without the stable mate. When we returned, I came home a different way from the other rider, and he was fine going up to the barn. Meanwhile, my other horse was yelling in the paddock.