This page demonstrates how to use the Coordinate Transformation Matrix property to map an absolute device (e.g. a graphic tablet's device like a stylus) to a specific monitor in a multimonitor setup. This method works on a per-device basis, so for a Wacom tablet additional devices may include the eraser, cursor (Wacom tablet mouse), and touch. See here for Wacom-specific notes.
Note: This page describes how to map the tablet to a monitor using the xinput command. Any such configuration will not be persistent, unplugging the device or restarting the server requires re-configuration.