I know you said you didnt want to get bunk beds so they can use them in seperate rooms later on, but what about loft beds? It would give them so much more room ...... you can make the bottom bed into a "L" shape ane then later on they would still look good in seperate rooms?
Ikea rocks for these - but personally im not a fan of their matresses. I got my matress from a dial a matress place in the local paper, and its the best matress i have ever had, for a grand totoal of $250!! Its bright red but who cares??!!