I think it is a wonderful idea. My two boys have always shared a room. They are now 12 and 14 years old, due to hormones coming into play we are thinking of splitting them up, but have not as of yet. Unlike you, we do not have a guess bedroom to give to one of the boys and will have to move one to the basement, if it comes to it. But for now let them build their bond as brothers and take it from there, you can always change it later.
Hope this helped.