How would you go about it Congo?
I'm not the one to ask .
Cameron suggested a Matrox card which I'm assuming has a dual display output.
You can use a PCI video card in a PCI slot as a second card.
You could possibly use onboard VGA as a second display if your bios allows addon cards and onboard to run at the same time.
Some setups require interfacing with FSUIPC.
JBaymore would know exactly what to do, as would quite a few guys in the homebuilt cockpits section of the forums.
I'm sorry town, I missed the fact that you already ordered the SLI board, that's fine, but you don't require the SLI compatible video cards for what you are trying to do, but you may as well get SLI compatible cards for performance useage when you want that aspect of SLI.
Simply ask the vendor if the card is SLI compatible to find out. Or post the card of interest here so we can check if you can't find out for some reason.