Opengl and PCI-Express

This is probably a OT, but i couldn’t find a valid answer on other web forums.
One problem i have developing opengl apps is to make them compliant with both Ati and Nvidia cards. Will it be possible to install one nvidia and one Ati card (both pci-express) and run the opengl app chosing the rendering device ? Or maybe to render simultaneously on both cards ?
At the moment i have two computers (one ati and one nvidia based) to make my tests, so the above solution (if possible) would be very welcome.

Assuming that you can install two different PCI-E video cards:

On a Windows box, you could enumerate the display devices, and create your OpenGL window on the monitor being driven by your card of choice.