Can USB act as a video output??

*As far as I am aware:*
you are able to utilize USB ports as video outputs.
With a basic graphics card you can only output either:
2 separate signals for split screen from the 2 standard dvi provided outputs.
OR
2 separate signals for split screen from the 2 USB ports.
NOT BOTH providing 4 outputs.

QUESTION
Does the USB port output video via the 2 graphics cards...thus providing only 2 outputs?
What are the ins and outs of dual layering graphics cards to provide further video output options?

is this expensive? does it require a new graphics cards? can i just add a further graphics card to provide 3 output for an extended screen display??

I hope I have provided enough food for thought.....Im very confused with how this all works
Any advice, help or 'know how' will be very appreciated..