Using the Onboard VGA output with a PCIe video card. Both nVidia

Not really at the same time... Each video card has a Bios in it, trying to take control of all video in the computer. Only one will win... Unless both videos are designed for sharing control (Crossfire or SLI), you cannot run two video cards together at the same time....

the only scenario that i know of in which both will work at the same time is ATI hybrid crossfire "hybrid graphics"

http://en.wikipedia.org/wiki/ATI_Hybrid_Graphics