We only have two breeding stallions right now, a donkey jack and a miniature horse (31") and they are seperated because the jack is very aggressive during breeding season. Back three or four years ago though, we had two other stallions (one has since passed from colic and the other was sold) and we had no problem with pasturing them together. When we first introduced them it was with a 5' tall sheep-fenced (so they couldn't take a bite out of eachother) in between them. We let them sort out their differences in there for a week or two, then turned them out togther in a 3 acre pasture. That gaves them enough room to chase eachother around without anyone getting cornered, and we never had a problem with them. They didn't fight, didn't play, really didn't even hang around eachother. However, they were pastured far away from all of our mares and were checked twice a day for the entire time that we had them together (I think about a year).
If possible though, I would advise in not risking it. As someone already said, stallions can and will fight to the death if given even part of a reason to.