iOS can stream audio to any AirPlay destination, but it can't stream audio to multiple AirPlay speakers like iTunes can. This little workaround with streaming app Airfoil solves that problem.
For those that don't know, Airfoil is a $25 app for Windows and Mac that streams any audio from your computer -- not just iTunes -- to AirPlay-compatible speakers. It's handy on its own, but this workaround gives it another clever use for getting around the limitations of iOS.
You'll also need its free companion app, Airfoil Speakers, which turns your computer into an AirPlay destination. Then, on your iOS device, just stream your audio to AirPlay Speakers on your computer, then stream that audio with Airfoil to multiple destinations. It will add a little extra latency to your music, and there are simpler ways to do it (like playing it from iTunes directly), but if you absolutely have to use your iOS device, this is a suitable workaround.
Note that I've had trouble playing to multiple speakers with Airfoil in the past though, so you may want to test this out with Airfoil's free trial before you go and buy it up.
Check out Airfoil's full how-to guide below for the details on the process.
Send Audio From iOS to Multiple Speakers With Airfoil [Rogue Amoeba Blog]