Pen display stylus points at both monitors, not just the pen display

Just tried connecting a Gaomon PD2200 pen display to my Debian-based x64 system, and it almost completely works right away: the display shows up as a second monitor, and the stylus moves the mouse. Unfortunately, the stylus moves over *both* monitors: if I set up my primary monitor to be above the pen display, then moving the stylus to the bottom of the pen display moves the mouse to the bottom of the pen display as it should, but moving the stylus to the top of the pen display moves the mouse to the top of my primary monitor. The mouse pointer is only under the stylus when the stylus is in the bottom right hand corner. What I'd like is to associate the input device with the monitor somehow, so X knows that all coordinates from the stylus should be mapped to mouse positions within this secondary monitor. But I can't figure out how to control this; the "Graphics Tablet" setting just says "No tablet detected", and I can't find anything relevant in the "Display" or "Mouse and Touchpad" settings. Does the setting I want exist? What are some useful search terms for this? I'm happy to make like it's 1997 and get down and dirty with the xorg.conf file.