Last time I let 2 of them duke it out one ended up dead.
What are you feeding your horses?! Sorry, I know that isn't funny but really.
I will echo removing the shoes and put them together. Make sure they are fed using 4-6 separate piles of hay quite spaced from each other so there is NO reason to fight over food. I would over feed them for the next week also. Two water tanks & no halters on.
Also, training wise I know Clinton Anderson has a technique that is to fix buddy sour horses... I wonder if you could reverse it to make them deal with each other? Its... two horses/two rides, ride the horses away from each other (Say 20 feet) and let them stand quietly looking at each other, ride them back towards each other and make them trot circles (IE work when your near your buddy, rest when your away from your buddy) ride father apart each time. Anyways it just popped into my head, I wonder if you reversed it to stand when they're near each other but work (trot circles) when they're apart if they'd get the picture... just a thought.