I have a friend who had her children's rooms painted by a local talented homeschool mom. She has three daughters under 5 and each one has a different theme. Maybe instead of choosing thematic bedding, you could meet with her (once you know the sexes) and have her paint the room and then use simple bedding to compliment. If it were b/g you could do a safari/jungle scene with blues/green with girly and boyish animals painted on the walls. I've seen her work with that and it was very cute. If they're same sex then the possibilities are endless. They use the crib bedding for such a short time. Although, most people I have spoken to keep twins in their cribs as long as possible to keep the rif-raf to a minimum at bedtime.
I really like the Baby Noah and Baby Grace!