Take a look at the Display Area Calculator included in the Tobii Gaze SDK package. You find it here: tools/displayareacalc.html. You should be able to calculate the three corner points for your active display area by entering the values for your combined screen plane including the two display monitors and the eye tracker’s position. You can then set the values to the eye tracker by using either the Set Display Area tool or the corresponding API call.
The EyeX Controller eye tracker works for screen sizes up to 27″ when the eye tracker is mounted centrally on the screen plane, and the user’s eyes are at about an arm’s length distance from the eye tracker. The precision and accuracy of the eye-gaze tracking drops notably outside this screen size.