Sorry to answer my own question, but a bit of research revealed that
while Xfree86 3.3 knows only one pointer, gpm can manage multiple ones
for it. My /etc/gpm.conf now has:
device=/dev/psaux
type=ps2
append="-M -t mman -m /dev/ttyS0"
and the pointer section in /etc/X11/XF86Config is:
Section "Pointer"
Protocol "MouseSystems"
Device "/dev/gpmdata"
Emulate3Buttons
EndSection
Cheers,
Stefan