This is now possible!
WHAALE is way outdated by now. I had asked a similar question here and I have found the best solution to be something called audio|acacia by a group called Plethra.
audio|acacia lets you play music from your iTunes / iPhone / iPad / iPod / Android Device / Internet Radio / etc. to any AirPlay device including your AirPort Express / Apple TV / iPhone / iPad / iPod / Computer / etc. Not only can you play to all of them, but you can play to all of them and have them be in sync with each other. You can also play one song to a group of speakers and a different song to another group of speakers at the same time. It's pretty awesome.
So that means you CAN play music from your iPad and you AirPort Express simultaneously.