In XFree 3.x, additional pointing devices, like mice or tablets, are defined
through the use of an Xinput section. The primary mouse is
defined as usual:
| Section "Pointer"
    Protocol    "PS/2"
    Device      "/dev/mouse"
EndSection | 
In the case above, it's a regular three-button PS/2 mouse, which works for the
trackpoint as well. Now, for the USB mouse:
|     Section "Xinput"
  SubSection "Mouse"
    DeviceName "USB Mice"
    Protocol "IMPS/2"
    ZAxisMapping 4 5
    Port "/dev/input/mice"
    Alwayscore
  EndSubSection
EndSection | 
My mouse is a wheel mouse, thus the protocol is 
IMPS/2. ZAxisMapping line is also needed for the wheel to work.
I also want the mouse to always work alongside the other one, and this is achieved
through the 
AlwaysCore option. If. for example, this was 
a serial mouse, the 
Port would read 
"/dev/ttyS0" for 
COM1.