Previous hints have (such as this one) have mentioned that iTunes now has a Multiple Speakers option if you have an Airport Express. An even quicker method to access this is to hold down shift while clicking on the Speaker menu. If you select a speaker that is unchecked, it will start streaming to that speaker as well as to any other that you previously selected. If you select a speaker that was already selected, it will deselect it and stop streaming to it. (It won't let you deselect if it's the only selected item.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060502195213736