I seem to have the same issue. Running two 660Ti's with two monitors on Windows 8.1. When I have SLI enabled I get the same FPS but both cards load up similarly. I also start to have system hard locks when playing FFXIV, or the display driver will crash and sometimes I can recover, sometimes I have to reboot. Turn off SLI and the same FPS and I can play without having the system lock up either. I play with SLI enabled in other games and have never had any crashes or issues yet, so I'd like to eventually see this resolved though I doubt it will be anytime soon.