We are about to do something like this... 3 1/2 year old daughter sharing a room with then-to-be 16 month old boy. So I am not speaking from experience. But I have noticed that when my daughter is asleep, I can go in the room and do quiet things and it won't bother her. When my son is asleep, same thing.
I would say, stop stressing about it. Stop going in to fix little things the moment they happen. Kids usually sleep deeply, once they get there, and can actually be hard to wake up. I've gone in to cover my 3 1/2 year old up after she went to sleep, and she didn't even notice. Or if she did, she only half woke up, saw I was there, rolled over, and went back to sleep. When my son's asleep, I've been known to do sewing in his room (I'll be so glad when we move and I can have a proper sewing room again!) and it didn't bother him. I just made sure he was deep before I started. And not near the end of his nap.
Honestly, I think they'll adjust. Don't stress about it. Just think... there are people with 6 or 8 or 10 kids, and they put 3-4 per room, and everyone is fine. It might take a few days to adjust all the wrinkles (such as crying a little--that's how I got my kids to sleep through the night and I highly recommend it for such things), but soon everything will be fine.
Good luck!