Siblings Sharing a Bedroom
First, make sure each child has some space that is her own and can't be invaded without permission. Each one's bed is her own "territory." One can't flop on the other's bed unless invited.
Even if they share a dresser, give each child her own drawer(s) that can't be opened by the other. It's a place where she can keep things that are important to her and they can't be borrowed or taken without express approval.
Let each child decorate her side of the room, and don't worry if the decor doesn't pass interior-design muster. Better to have each one feel like she can express herself in her choice of comforter and wall posters.
Give each one her own lamp so she can read, even if the other wants to go to sleep. Eyeshades can help if either has trouble sleeping with the light on. Individual sets of headphones mean that neither has to listen to the other's music choices.
You can't change the dimensions of the space, but you can give each child a sense that she has a place to call her own and avoid sibling rivalry.