SLI and crossfire were intended to have more than one GPU render a single screen or single monitor.
back when they were created, they didn't have enough GPU power to render the full high def screen 1920x1200 with a full speed game.

SLI/crossfire has half the picture on the alternate GPU and then ship the 1/2 image to the main card to be merged with it 1/2 image.

Once you place two monitors in the same setup, it's not any different than having one GPU do one screen and the other GPU doing the second screen ( what's the point of having each GPU do 1/2 of each monitor, when they can do 100% of their own monitor )

you might as well turn off SLI/Crossfire and plug one monitor into each seperate card and hope for the best.

good luck