M.T.
My oldest son and middle son are the same way. Always have been. They are now 21 and 18. They live in the same house and now they at least ignore each other. But it has never gotten any better and I don't imagine that it ever will.
I think that my oldest is jellous of my middle.... and for no justified reason. It seems that the middle one should be jellous of the older one.
So I don't know what to tell you. I think that me and my X were so different that one boy took more after me and the other took more after his dad so they definitely CLASH BIG TIME!
But it has never gotten better. All I can say is that hopefully they will grow up a little and learn not to fight.
Good luck. I have searched for that answer for years.