There may be many reasons for siblings to share a bedroom. You may not have enough bedrooms for them all to have their own room, you may wish to free a bedroom for guests or use it as a study. You may just wish to have your children share because that is what you did as a child.
Whenever children share a bedroom there will of course be times when they both dis-agree and as they get older they will become more aware of wanting their own space and privacy. As long as you as a parent are prepared to understand how your children may find it difficult sharing the space and need time alone then you should be able to make room sharing work.
Room sharing is more difficult where there is a big age gap between the siblings, if sleeping patterns differ between the children then one may disturb the other. Siblings who are fairly close in age (2-3 years) will generally develop a good friendship sharing a room.
Positives to room sharing
Children sharing a room will learn to share, to compromise and to bond with their sibling.
Negatives to room sharing
Personalities may clash and arguments may develop over space and privacy.
Try and agree a theme with the siblings for decorating a room. Agree colours and if possible get them involved in the decorating as part of the agreement. If possible allow each sibling their own set of drawers and a hanging space so they have their own area to keep their possessions.
Mixed sex siblings can share a room, the sharing should stop when you and they agree they are not happy to continue. As teenagers reach puberty they will naturally want their privacy and you should do all you can to ensure all siblings respect this.
Set up some rules for the shared bedroom. Each sibling should knock before entering the room and allow their brother or sister time alone in the room. They must respect each other's belongings and keep the room as tidy as possible.