You don't. At least with the standard mouse drivers, Windows doesn't handle multiple mice, so your app can't do it either.
You can plug multiple mice into the machine, but you only have the one mouse pointer. If you move both mice, they will "fight" for control of the mouse pointer.
The only way to get multiple mouse pointers is if you use mouse drivers specifically written to handle this. For example,
CPNMouse[
^].