I might be misunderstanding you, but as far as I know, GPU is nothing like CPU at all, in other words I didn't heard of assigning specific GPU to render certain application, well not in way CPU core can be assigned...

usually second core working when that card set in SLI or Quad-SLI modes, I believe ASUS with 4xDVIs have second core rendoring 3rd and 4th screans if you have them connected, at least that what I learned here and there.
also in SLI you can have only one screan connected to a card, dont know when they take that limiter away >.>

BTW What OS do you have? What DirectX version? What card do you have?