Some key things to consider
Room-sharing can be tricky and the specifics of an effectively designed shared room depend on the kids in question - their ages, temperaments and tastes. In all room-sharing situations, though, there are a couple of key areas to consider:
Layout
Everyone needs their own space, so it's helpful to think of the room in three zones - two private (one for each child) and one shared. The areas may be quite small but it works best if each child has somewhere for their precious things and to feel is just for them.
Storage
Double the kids=double the stuff, so storage space must be carefully considered.
Design & style
A pleasant room doesn't have to be matchy-matchy, but aesthetic elements do have to work together if you want a great looking bedroom! This can be ore challenging for children of quite different ages or a boy-girl bedroom, but is not impossible!