This is how to configure a USB-Mouse for parallel use with the touch-screen and the track-point at the same time:
SubSection "Mouse"
DeviceName "USB Mice"
Protocol "IMPS/2"
ZAxisMapping 4 5
Port "/dev/input/mice"
AlwaysCore
EndSubSection
Here is the updated version of the XF86Config, which is ready to work with the USB-Mouse as described2.