You can combine AirPlay in iTunes 11 and up with something like AirSever to accomplish this with good results and very little effort. (Note: For more options on how to turn your Mac in to an AirPlay receiver please see: Can I use a Mac Mini as an Airplay audio receiver?)
Install the AirPlay receiver software of your choice on the machine you want to stream audio to.
Start iTunes on the machine you want to stream audio from. In the upper left hand corner of iTunes, right next to the volume slider, you'll see the output device selection icon. It looks like this:
Press it and select the "Multiple" tab at the top of the screen. Select both "Computer" (which is the computer you're running iTunes on) and the name AirServer machine from the list.
Play something in iTunes and should play, simultaneously, on both machines.