I swear I had found something while I was in college regarding having two mice in use at once. We were thinkign about using it in a game environment but never did. I forget the name of the program but a quick google found this.

http://cpnmouse.sourceforge.net/

I'll keep looking to see if I can find anything else on it.

found this as well:

http://sourceforge.net/projects/dualmousekeyboa/