J.C.
put the younger one right next to you, and then when a new baby comes it will go in there with her when you are ready ( if you are cosleeping it might not be for a while anyway...) A family I babysat for when I was younger had 2 girls and a boy, all about 2-3 years apart and they switched the room arangements from time to time as the kids grew up and none of them were shaken up about changing rooms. at one point the two girls (oldest) were sharing while the boy (youngest) had his own room. later the oldest girl was given her own room and the younger two shared.
It might be different for you because of the age difference, but I don't see that as too big of an issue. I would just want the younger one closer.