Windows should have no problem recognizing multiple cameras. As a matter of fact, it prompt you to download as soon as it recognizes that a device has downloadable medium (in this case photographs), be it a camera or a USB stick. The fact it doesn't see your Mark II indicates that there's no active communication between the camera and PC. This could mean that the camera is not turned on or there's no driver for it (at least the PC doesn't think so but it does). Sometimes the PC gets interrupted when you plugged the camera in and so it does not recognize it. What you'd want to do is to plug it in using a different PC USB port and wait for it to go through the whole thing. Most of the time, this will do the trick.
