For some reasons, most native games as well as Wine have a problem with Twinview. So when starting SC2 I have to manually disable one of my two screens in nvidia-settings. (By going in X Server Display Configuration > click on second monitor > Display > Resolution: off)
I searched hard but couldn't find a way to do that automagically. nvidia-settings itself has non-GUI options (see "nvidia-settings -q all") but none of them seems to do what I want. I want to put that in my startup script for games, which already replaces Compiz with metacity (and back when it exits).