Need help: how to get 2 boys to share room?

[Wondermom]

I know tons of you have kids sharing rooms, and I need advice on how to plan/make the move for my sons. My younger son is 15 months. My older son will be 4 in August. I'm afraid the baby will lose a lot of sleep if we make the move (and so we will lose sleep), but I hear it can be done.

Right now, the baby's very good with a routine--bath, bottle, cuddles, bed--sound asleep for the next 11 hours. The older son is a boisterous ball of energy who takes at least an hour to wind down. We read books with just the bed-side lamp on, and then lights out, but he's often awake and singing, talking for a long time, and will usually get up out of bed at least once (if not several times) using the potty as an excuse. It's not unusual for his little brother to have been asleep for an hour before the older one finally falls alseep. Then he's usually up and awake at least 30 minutes (to an hour) earlier in the morning--usually to potty, so I don't blame him, but he's never able to go back to sleep afterwards. With their sleep habits so different, how can I make this work? Or should I just hold off on moving them in together and go longer without a spare bedroom for guests?

Also, logistially, do I train baby to sleep in a big boy bed first in his own room? Should I use the twin bed that he WOULD be sleeping in if and when he moves in with big brother? Or train him in the full-sized guest bed that's currently in the nursery with his crib?

Thanks in advance!