The best way that I've seen CATIA used with multiple monitors is to run CATIA on one, and other applications (e-mail, IE, Excel, etc) on the other. It's especially nice when you have the Help Documentation on one screen and CATIA on the other.
You can make CATIA span both monitors, but you tend to lose things in the center - like when you re-center the window, it will center between the monitors.