What happens if the user sets a custom resolution? Like 1920x1080 or such. If that works (/only shows on 1 monitor), than it might be an idea to add an option where users can set their amount of monitors and than do something like this;
video_set(sys_metrics(0) / monitors, sys_metrics(1), 32, 1);

(probably doesn't work if the connected monitors have different max resolutions though)

Another idea might be to take a peek at msdn and see if there is some windows function that gives all the monitor's resolutions or such.