FYI, i did find a way to play two seperate song files at the same time through one computer. I just loaded Net Tunes to control one computer remotely. Then from computer #2 I loaded Itunes and selected my airport express as the target. Seems to work fine, one computer playing two seperate songs, but not one computer playing the same song through two stereos...I guess a trip to the wire store, a bunch of holes in the wall ought to solve it.
I recently got a M-Audio Firewire Solo. I have the line outs going to my amp in the computer room and a 50 foot RJ6 from the SPDIF out to my amp in the living room. Works flawlessly with no sync issues and is a dramatic increase in sound quality from my quicksilver 733s headphone outputs.
2.66 x 4 Intel Mac Pro, 4G RAM, 160G System 500G Scratch, Media, 320G Photo Capture, 320G Video Capture. 733 Quicksilver 1G Ram 120G system 60G backup 160G and 200G capture.
You can do this with multiple Airport Express units. See Rogue Amoeba's Airfoil. Also, macosxhints discussed this issue here. And from Apple's Airport Express webpage: "Imagine being able to play your iTunes music on whichever speakers in your house you prefer —- or all at once."