Your best bet if you do prefer individual controls would be to get something like a nostromo or one of the similar devices mentioned on these pages, and use that for the one or two buttons. (Or use it for the main and the keyboard for the alt.) Use software like Keyclone or AHK to actually send each set of keys to a seperate client.

If you use the nostromo, you can even set a shift state up so that you can change which client you directly control without taking your hands of the device.

That won't solve the issue of the mouse, Sadly I don't know how to make two mice function on one PC (not that I have ever wanted too )