You obviously have the correct Video Driver.
Within Windows, the "x,y" DISPLAY co-ordinates of Monitor2 are at "(w+x2),y2" to extend Monitor2 from the R edge of Monitor1 (x1,y1). [w is the width of Monitor1 = x1 at its R edge]
Within the Driver the TOUCH co-ordinates seem to be individually reported (from the IR sensors) as "x1,y1" - causing Monitor2 to report IR co-ordinates that are actually on Monitor1.
I have a feeling the Dell Driver needs to be 'cleverer' at reporting Touch on Monitor2.
You could try and "Calibrate" Monitor2 (if that's actually possible) using "Tablet PC Settings" in Control Panel - be prepared to "Reset" if it gets weird.