M.G.
I have boys that are 3 years apart and they do argue alot and have pretty much since the younger one got old enough to know how. Neither have had their own space, they have always shared a room.
I discipline them both. I don't want to hear details as to what, where, when and why, they both get punished.
I think the experts would have you teach them how to relate with each other and teach them how they are to treat and respect others.....well we are way beyond that. They get disciplined and then I have them tell me why they got in trouble and they usually know the exact reason...so they know I don't have to teach them all that. They are chosing to do it because they think they will get away with it. They know better than to do it at school so they know better at home.