Can i combine My Graphics Card with onboard video?

I wanted to know if there was a way if possible to run my onboard video. Intel GMA 3100 with my EVGA Nvidia 8400gs graphics card, almost like in SLI configuration. Or could i get two PCI graphics cards and run them in SLI? My GPU runs at 522mhz and i think onboard video runs at around 300mhz or higher. I want to know if you can combine the two.