We had dinner the other weekend with friends who have a 3 year old daughter and are having a boy within the next month, and they'll have to share a room for awhile. It seems easier to have same-sex room decor, I myself have both boys in a Spongebob-soon-to-be-Star Wars room. So I did a little digging for some ideas to help some of you out with this decor dilemma.
BHG has a great slide show on shared kids' rooms. Although they are mostly boys'/girls' spaces, a few are neutral enough for mixed doubles. I've picked out a few favorites, but check out the slide show and mix and match some ideas to suit.
I always wanted to have a set of twins, one of each, and kill two birds with one stone. It's a great decor idea to have each child's room half decorated to their own tastes or color and tie it together with a coordinating wall color.
Primary colors like these two rooms can easily grow with your pair. Use complimentary "traditional" boy and girl colors, or keep it neutral. Or try a gender-neutral theme like this nautical room and this bubble-dots.