Mouse setup under X

[Paulo Lopes <pmml@netvita.pt>]

Hi, i've a laptop with a touchpad that can be easely configured on X, my 
question is how can i configure it to enable an external mouse WITH 
scrollwheel. The problem is that both the touchpad and the external 
mouse are PS2.

my current config is:

Section "InputDevice"
        Identifier      "External Mouse"
        Driver          "mouse"
        Option          "CorePointer"
        Option          "Device"                "/dev/psaux"
        Option          "Protocol"              "PS/2"
        Option          "Emulate3Buttons"       "true"
        Option          "ZAxisMapping"          "4 5"
EndSection

in order to enable the scrollwheel i've to change the Protocol to 
ImPS/2, but then, if i've no external mouse on /dev/psaux (i'm using the 
touchpad) it's impraticable to use it, because the mouse pointer is 
always junping to the lower left part of the screen...

Can someone tel me how to setup two mouses on the same device, using two 
diferrent protocols?

Thanks!
Paulo.