Extend game to 2 monitors

Are there ways to extend a game to use 2 monitors?
I’m thinking in use one monitor just for the game, and extend all the menu, inventory and additional stuff in the other monitor.

I want to run some Unity scene at a resolution of 2560 x 720, and extending the scene to appear in both monitors.

Is that possible with unity3d or some indie library ? Or is something hard from the hardware perspective?

Unity 4.1 has exactly this feature. See the Display class. (Note, this is a bit basic at the moment.)