the SLI will not help you with this setup.
SLI is intended to have two GPU making up one screen of data back when one GPU couldn't or had a hard time rendering 1920x1200.
once you connect a second monitor you pretty much defeat the whole purpose of SLI.
if you must put two cards into your machine have one monitor on each card, disable SLI and hope for the best ( hope that each card renders it's own monitor as opposed to the main card doing all of the work anyway ).

I agree with the other poster though, most video cards like the 285 are so awsome today that one card should be able to drive both of your monitors.
I use a 280 and have two monitors at 1680x1050 but I only run 3 copies of wow.