I would say do one wall color that they both can agree on and have things for the boy on his side near his bed and things for the girl around her area. Like if she likes certain colors, use whose colors for her accessories on her part of the room, also make each a designated area in the room using the colors and thing that are theirs, whether it be teddy bears, dolls, and such. display their toys that are nice and put the others in storage trunk at the foot of their beds or in in a storage shelf of some kind. Have plastic boxes in one color for one child's things and boxes in another color for the other one. You can even make each a desk area of their own to do crafts or homework and such. You can be as creative as you want with it. The website
www.hgtv.com and the
www.diynet.com website might help you with some ideas for kids bedrooms/shared bedrooms. They haves some great ideas for decorating kids rooms. Hope this helps you. God bless.