C.D.
Staggering bedtimes didn't work for us so here's what did. Both of the kids were very excited to share a room but we told them that they would only be allowed to share if they helped each other go to sleep at night. We especially appealed to the older child to be a good example. We have a pretty set bedtime routine and then they are expected to stay in their beds although we do allow them to talk. It took a week of constant vigilance at the door telling one or the other to get back in bed before it really stuck. But since then we rarely have a problem.
You can make it work!