I would chose a nice solid color for the wall, without wallpapers or trim borders that compliments both themes. Then over each's bed and corner of the room, use just those items that actually match the others theme. Like if there is yellow and blues in both, pick up on these two.
If this is a temporary living situation, put the other items away in storage and use later or with another child. Too much eye clutter makes the room feel too busy and looks cluttered rather than tied together and relaxed. :)
Don't be afraid of using color on the wall, but be sure if you aren't going to be in this room for long, that you don't use something that has to have multiple coats to cover. New owners or renters hate that. Find a nice muted color that matches both themes. :)
at 1:06 PM on May. 1, 2009