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. The following example is for a PS/2-mouse:
Section "Xinput"
SubSection "Mouse"
Protocol "PS/2"
Port "/dev/mouse" #could also be /dev/psaux
DeviceName "Mouse"
AlwaysCore
EndSubSection
...
EndSection
Several people pointed this out, especially the usage of the
AlwaysCore option. Thanks to them all.