To use both prod the Nvidia control panel and enable SLI, though you'll not get as big a boost as you expect, probably only around 30 - 40%

*EDIT*

Seems you need a bigger PSU to do it, the normal one (120w) apparently doesn't have enough juice. Gotta make sure it's a 170w one or it'll refuse to work. It'll also disable it when it's running on battery.

/moved thread to a better place.