I was reading Elizabeth Pantley's book last week (the No Cry Sleep Solution), she suggests a sibling bed as as alternative to a family bed. These two little buddies sound like perfect candidates for something like that.
On the other hand, what will you do when the older one is a teenager, and wants some privacy and someone has to explain to the little fella that he has to move out?
If it was me, I think I'd get them to share, but after a few years go by, I'd start casually mentioning to the little one that big boys have their own room and get him a bit excited about moving back out again. The ideal scenario is that the little one moves back to his own room of his own accord before the older one wants his space back.
Keep us updated, I'd love to know what you end up doing.