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 ... EndSectionSeveral people pointed this out, especially the usage of the AlwaysCore option. Thanks to them all.