I love my dual Screen, So helpful and very much a time saver.
With Full Screen Games, its different for all, some black out the monitor there not on as others literally just take one screen and the leave the other.
I do this with strategy games, and leave guides on the right monitor while gaming on the left. Of course to scroll the page on the guide requires a simple Alt-Tab out of the game for a few seconds, but with the right PC this should crash or affect your game.
I use AMD Catalyst eyefinity to make my two monitors One, and this allows you to game on both monitors instead of one (unless the game has built in dual screen support) One problem with this is that shooter + driving, the place your aiming is in the join of the two displays. Which is why im thinking of getting a third.
I have always dual screened on identical monitors i find it more easier on the eyes.