Airfoil for Mac: Play Audio on Multiple AirPlay Devices Simultaneously
In response to the previous commentor, I didn't get this problem using my Mac.
In addition, you can now use Raspberry Pis as additional airplay devices. I now have three audio sources scattered about the house, and it's pretty awesome when I start tidying the house (note the when).
Here's the link I followed:
Only thing with the Pi, if you're using Shairport (which I recommend), make sure you fiddle with the frame buffer (-b). I'm playing with values >= 240. They seem to be about right, other wise the Pi plays before everything else. Not sure if that setting will be different depending on your network quality or whatever though.