I've seen a few tips on different sites explaining how to run two copies of certain applications. Most of these involve creating a second copy of the Application bundle, which can be impractical if it contains too many resources, and is a hassle in any case.
Here's an easier way that I don't think anyone has posted here: Just run the application that you'd like to launch a second time from Terminal, using the open command with the --new (-n) flag. This will force an additional instance to be opened regardless of how many are already running. For example, if I wanted to run another copy of Mail, I'd do it this way:
open -n /Applications/Mail.app
Remember that you can drag an icon to the Terminal window and it will insert its path, making this even simpler!
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008040521371015