This makes me cringe. I have to ask why (out of morbid curiosity)?
Now I will encourage this:
http://www.iogear.com/product/GUC2015V/
This would be what you are looking for or something like this.
The setup:
Connect 1 monitor with an adapter to your mini-display port
Connect 1 to the USB video card
I will never understand why people do this but there it is.
Cheers