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.