You can't have two pointers at once in the GUI (at least not natively), but two mice can control the same pointer, just as two keyboards will control the same input. It does work, but not quite how you're looking for.
However, I do believe that with a certain few extra-smart mappable DirectInput applications, you CAN use them as independant devices in those apps.
Oh, and Qumahlin ... it does depend on the device (and your USB bandwidth load), but you can get MUCH higher resolution out of a USB mouse than with PS/2, so I wouldn't exactly call USB a waste in that particular circumstance. It may be said that it's a slight overkill in keyboards, although USB keyboards are a bit more hot-swappable (some spaz out when removed under PS/2), and HID-friendly keyboards work a bit better out-of-the-box under USB. Also, many USB keyboards offer built-in low-power USB ports, so blowing a port becomes less of an issue. Besides, as parallel and serial ports become increasingly obsolete (they pretty much are now anyway) and USB hubs become cheaper, I see little reason NOT to buy a USB device these days.