To use 2 card at the same time, they both have to be of the same brand to work. You can't mix up an ATI with an nVidia, that won't work.
For installing the new card, you habitually have to remove the old driver. Remove the card from the computer, install the new card and reinstall the drivers. Simple as that.
But if you still choose to use both card, I can't really say since I never done it. But I suppose that when you'll install the new one, the computer will detect it and do what's need to be done to make it work. Since they both need to be of the same brand, you'll already have the driver installed so you shouldn't need to install them again. It should be pretty easy. But like I said, I never done it, so it's pure assumption