I am not sure what setting to change for this but I open games like Dungeon Crawl or a Steam game and the game wants to take up both screens. Is there a way to have the game open just on one screen xrandr shows this. So they are both on Screen 0:

Screen 0: minimum 320 x 200, current 2966 x 900, maximum 8192 x 8192

LVDS1 connected 1366x768+1600+75 (normal left inverted right x axis y axis) 344mm x 193mm

VGA1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 443mm x 249mm

If you need any more info or can point me to a place you may have already found an answer please let me know.