In some cases, for instance a laptop with a built-in pointing device,
you may wish to use a serial mouse as a second device. In most cases
the built-in device uses the PS/2 protocol, and can be ignored if
you don't wish to use it. Simply configure gpm or X to use /dev/ttyS0
(or whatever) as usual.
To use both at once, you can use gpm -M
to re-export the
devices. More details in the gpm man page.