You can use one keyboard, one mouse, and one computer on multiple monitors. You will also only have one pointer.
So, you can run multiple programs at once, just like with one monitor. Excpet if you want you can drag programs to one montior, and work in other programs on the second monitor. Or, you can put all your photoshop tools and layers on one monitor, and the image on the other.
You can even stretch one application so it fills both monitors.
But you definately can't have two people working at the same time on two different keyboards, mice, and monitors with one computer.
I don't see why you couldn't have a monitor dedicated to running your Mac programs and the other which you use for Virtual PC. It's just that you can't actually work in both at the same time. You could have iMovie or iTunes rendering or playing while you are actively working in AutoCAD in Virtual PC. As long as your computer can handle the workload.