Conflicting Mice

[CornedBee]

I have Gentoo installed on a laptop, which I describe here:
https://forums.gentoo.org/viewtopic.php?p=561487#561487

I have some sort of conflict issue between the internal touchpad and my external Trekker wheel mouse. Both are available through /dev/psaux.

Whenever I used to start X with the external mouse attached, the cursor went wild. It jumped to random places on the screen and clicked everywhere, with me only moving the mouse a little.
I found that if I set my BIOS to disable the touchpad when an external mouse is present, it worked. That is, the mouse did what I told it to do. The mouse wheel didn't work.

According to the X manual on this page I managed to get the mouse wheel working by setting the mouse protocol to "IMPS/2" and adding a ZAxisMapping.
However the touchpad doesn't like this protocol and freaks out if I unplug the external mouse so that the touchpad gets activated.

My main question is: is there any way to configure the external mouse and the touchpad as separate devices that both access /dev/psaux so that I can easily switch between touchpad and mouse?
Even better would be a method where I can have both at once.

Thanks in advance.
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CornedBee