I bought the 3D vision stuff and sadly in my opinion its a waste of money. Very cool at first don't get me wrong, but down side it that you can NOT box while playing in 3D, or to be more precise you can't use 3D vision in anything other than full screen mode. Which stop the passing of keystrokes (keyclone). Also trying to click on things is a pain in the ass.

Too answer your question you only need 1 3D monitor. I also run a 2 monitor setup and only use 1 as the 3D.