For Mac systems that are capable of supporting more than one monitor the multi-monitor configuration is shown in the System Preferences, Displays, Arrangement dialog as follows:
In the above setup the UPDD configure function will associate a touch device with a defined monitor; Monitor 1, Monitor 2 etc
In this case UPDD Status will show the association between the touch device and the complete desktop/monitor:
In the example above the 2nd monitor has 2 UPDD controlled devices, touch and pen, associated with the same monitor.
External multiple head devices
External video extenders are available for the Mac such that a single video port, typically on a Mac Mini, can be extended across two or more monitors.
One such device, dualhead2go, or triplehead2go allows two or three screens to be connected to a single VGA connector on the Mac system. To the Mac system, it looks like one wide screen but its actually two/three separate physical flat panels.
In this case the one logical monitor represents two physical monitors.
Prior to using the UPDD Configure function to make the desktop / touchscreen association you need to defined the logical / physical relationship using the command line utility to set monitor_segment_map setting accordingly, i.e upddutils set monitor_segment_map 2x1.