You want to use two video drivers for one video card? If so then never should you do this. If your installing 2 sets of drivers the second set you install is just going to install over the first anyhow. This way your card is still going to only use one set. It's not designed to use two. Even though all your going to do is install over the other this is not recommended and there may be files left behind and you will have trouble. Always fully take off old drivers and clean what files are left behind before installing your drivers. You can clean out old drivers for Nvidia or ATi Video cards with programs like Driver Cleaner. You can find that via Google.
Point is never try to install two different drivers for any component.