Prior to the release of iTunes 9, iTunes would not allow two Macs to use the same iTunes library at the same time -- iTunes would report that the file is locked.
iTunes 9 (and 9.0.1) now allow two Macs to access the library at the same time! In my testing, I was able to run iTunes on both Macs, and play the same song, with no issues.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090929160156850