S.C.
I would start with naps in the shared room when you are awake and alert to any problems. This will get them used to being in the same room and give you an idea of how older child is going to react. I kept my now 22 month old in my room til she was over a yr old simply because her older sister would try to take care of her... get her out of bed etc.. One thing to think about was an issue with us- when I kept the baby in my room, I was the only one that got woke. When I tried to put my girls together when the baby cried she woke up the then 3 yr old and then she would not go back to sleep.