How to configure the device 3. Activating the device 4. Using touch screen and mouse in parallel 5. At first verify that the device driver modules exist. Now have a look into XF86Config.
X needs to know some parameters about the device.
The device driver reads these values and interpolishes the other coordinates. To use the touch screen and a mouse in parallel you need to enter the mouse specs in the Xinput-section. This definitions should be the same as in the Pointer-section. Several people pointed this out, especially the usage of the AlwaysCore option. Thanks to them all.