What operating system are you using? If Windows....

I use a single monitor, but I did try plugging a second one in (which killed my framerate, which is why I still use one). Had to restart Windows (XP), but I was then able to set up how the second monitor was used in Windows' display settings. You shouldn't need any special software - Windows just treats the other monitor as an extension of your desktop. I DID find that I couldn't run any windows in "full-screen" mode, as they wouldn't let the mouse pointer leave that screen. With all WoWs in windowed mode, everything worked, it was just clunky (not to disuade you - my cpu is getting out-of-date).