Getting them ready to share in the same room?

Our 20 month old boy is in the nursery (and in a toddler bed, we converted the crib), and our 3 1/2 year old boy is in the big boy room that has a bunk. The bunk is a twin over full size, and instead of a ladder, it has stairs that go up to the bunk. Big boy sleeps in the bunk, so baby would sleep in the full size bed underneath.

We would like to have them start sharing a room. Baby can get up the stairs, but not down, and we were even thinking of putting the converted crib in there to start before putting him in the full size bed.

Is there a rush? No, not really. They're 20 months apart, so pretty much wear the same size clothes, are both boys and play with the same things, etc. Before bed, they enjoy playing in the big boy room and when I ask our older son if he'd like baby to sleep there he says yes. I think I want to wait until the baby can comfortably and easily walk DOWN the stairs until we move him in there, but as we prepare for this next big step we were just looking for some advice, thoughts, etc. on how to successfully and happily move them into the same room, precautions you suggest, your stories.

Thank you for your help YA-ers!!!