You could use the proximity sensor too, most of the time the missing feature is a fire button. Also it could be configured to "hold" first finger position, and track the touchscreen input as second finger when "touched". If this feature is implemented not in game but preenv level, multitouch is given in all games. there could be a controlpanel config app to set the "fire button" position in every game or custom positions or configs. Secondary camera could also track slight motion (like half covered in different ways).