What is only 1 mouse working when two are connected?

Hi,

I have a system that is working fine with mouse 1 connected.
As soon as I plug in mouse 2 in addition to the existing mouse 1 this happends:
- mouse 1 is not working
- mouse 2 is working
If I reboot then (with both mice connected) this happens:
- mouse 1 is working
- mouse 2 is not working
If I then unplug mouse 1, again mouse 2 is working.

Can somebody tell me which settings in xorg.conf or somewhere else can lead to this behaviour?
I would like both mice to work as soon as I plug in the new mouse.

Thanks

J.