On the subject of bitter apple...I find that 50% of the time it does not work....I got some for my dogs because they were chewing up the chairs and had a friend that got them for her two horses who would chew up the fences
My dog LOVED the taste of bitter apple...infact the more I put it on...the more he chewed and licked the chair....
Where as the other dog (my sisters) hated it...and wouldn't even go near the chair
My friends mare hated the taste and wouldn't go within 5 feet of the fence.
Where as her gelding would chew away as if nothing was on there
I'd defiantely try it, but just to warn you that it may not work.
What I'd suggest to do it simply find a day when you have a good 3-4 hours...bring a book, and pull up a chair infront of the pasture or to the side of it. Just sit down, and read the book and watch your two horses. Hopefully (well not hopefully) the one will show some type of anger and try to bite...try to see what causes the one horse to attack the other...if it's a "get away from my food" thing then put the food into two seperate piles (if possible)
If it's a "you're too close" there might not be anything you can do. Just simply see what is causing the attack and that may help find a way to prevent it