See if either is compatible with either a passive (pin-only) PS/2 to USB (very few are) or PS/2 to Serial (common on older ball mice), then all you need is a really cheap adapter. Serial compatible mice may have a small switch on the bottom to switch between modes, but you're going to have to Google to see if a newer one is compatible with USB directly. I don't think you can Y them off the same port, but using serial or USB would work.
If neither has compatibility with passive adapters, you'll need a PS/2 mouse and keyboard converter (with circuitry inside). Here's a cheap one.