I'm going to say that they WILL fight, almost guarentee it! HOWEVER, if one of them backs right down and accepts the back seat, they may be fine. The big dangerous fights are usually when you have two who are convinced they are each the ALPHA. Sometimes one will just shrug after a bout or two and say, ok, you win. They'll still sneak around and breed a few hens when they can, but won't challenge the other.
Problem is, untill they work that out, you could be in for some bloodshed.
Twelve hens is a pretty small number of hens for two roos... more hens would mean the beta roo could get to some easier when the alpha is distracted, which would keep him happier. Not saying you should, or must get more, just saying why it might be a good thing.