ok I think this mite be, dominants, it seems to be Dolly is just showing whos boss, and Banjo is giving into it. For example, I have 2 dogs, Rosie and Shelby, Shelby is always done eating first, when she is done she goes over to rosie and snaps at her so she can get rosies food, and rosie gives in. So I had to fix my problem by feeding them far away from eacother with their backs turnedto eachother, that way they can't see eachother. And their fine now. For your horses imo I suggest you should seperate them when feeding and when one is dne before the other, go over to the one who is done and pet him and praise him, in other words, distract him while the other is still eating, and when their both done praise them, they'll be fine.
-chelsea- Good Luck