iPad Airplay and Multiple Airport Express Steup

I have two airport express stations set up on my network with speaker attatched to both in different rooms. Airplay is working fine on my iPad and iPhone 4 , but to only one set of airport express at a time. I can't steup multiple Airport Express at once. I can play one set or with iPod on iPad, also play one set of speakers, but I can't find any multiple speaker option. First of all, you can