You didn't mention what laptop you have.   The laptop hardware may be
managing the PS/2 port and the touchpad as a single device appearing
to the OS.

On my Thinkpad T30, the builtin trackpoint and touchpad are both
active by default, as one PS/2 port.  But, the touchpad has only 2
mouse buttons, so the third trackpoint button doesn't work.  Turning
off the touchpad in the BIOS made the trackpoint work fully.

Check your BIOS for settings to change how the builtin mouse and
external mice are handled.

-- 
        Greg Troxel <gdt@ir.bbn.com>