I wrote an article a few days ago on using AirFoil to do multi room audio like sonos does using hardware you might already have. It was pretty well received, read it here if you missed it.
However I didn't really explain the trick to playing to multiple speakers from an iOS device. This is totally possible, just requires one extra step.
You need to install one other application on your Mac (or PC, if that's how you roll). This one's called AirFoil Satellite/Speakers. It's a companion app for AirFoil by the same people. It turns your Mac into an AirPlay speaker.
Install it, and run it. You'll see AirFoil Satellite show up in in your list of AirPlay speakers, in both your iOS devices and in AirFoil. You don't want to tell AirFoil to play TO this speaker, you want to choose AirFoil Satellite as the SOURCE. Once you've done that, select AirFoil Satellite in the AirPlay section of the iOS device you want to play to. You should hear the music come out of all the speakers you have AirFoil set to play to. I rename my AirFoil Satellite/Speaker to be called "~Whole House~". This reminds me NOT to play to from AirFoil and the tilde (~) in the front keeps it at the top of my AirPlay speaker list in my iOS device.
A note about doing this, your iOS device (particularly iPhones, more so than iPads), need to be in good wifi when playing using AirPlay. You'll get much better results setting it on a table or something than if it's in your pocket.