For many families, it's just not possible to give siblings separate bedrooms. (I think lots of times kids are secretly glad to share a room anyway.)
When kids are very young, you may want them to share sleeping space and use the other bedroom as a playroom.
That way, they can comfort each other in the dark and have more space to play in when it's light.
Here are 6 ideas to help them share with less whining.
Give each child his separate space and privacy. If possible, divide the room into halves.
Try a folding screen by each bed or a curtain or bookshelves bisecting the room for really rocky sibling rivalries.
Consider ages when doling out room assignments to your kids. Shared spaces work better for younger children. Tweens and up want their own space.
Set sibling rules such as who can (or can't) touch what; never borrow each other's stuff without asking, etc.
Let each child decorate her part of the room the way she wants. This helps make the room a place where each child has ownership.
Give each child her own closet, even if it's a closet in another room. A rolling wardrobe works well in tight spaces.
Finally, designate sole ownership time. Let each child have the room to herself for 30 minutes each day. It's the easiest way to prevent sibling squabbling.