J.C.
I have 6 kids and 3 bedrooms for them. That being said we have one girls room and one room for the boys. My boys are easier to put down at night than the girls. The girls are up gabbing and such for a while, but the boys down they go and they are done. We (my husband and I) feel that it is better for them to learn to share their space and eventually it will bring them together as siblings. I don't remember when we started having a room sharing issue because we have always done it. Just let the children know when you start that this is the way that it will be and there aren't any other options. I think that the younger they start the easier it is. As another post said, the kids will sleep through the others crying and such in the middle of the night. Good luck, and congrats on the new addition.
J.