GRID doesn't work with Crossfire. I also use the Steam version and I never had to edit any files, just disable Crossfire. GRID is not very demanding on your GPU so it should run fine on a single card and you can also use bezel compensation.
Yeah I had the same issue with my steam version. Won't even launch with nvidia surround enabled.
I had to totally disable spanning in the driver to even get the game to launch.
Maybe it's just the steam version that's messed up? Because people here have played it on 3 screens and I've seen it on youtube being played in surround. :|