I have a KN8-SLi and I was curious about the following two things:
1. Can I put a second GPU not in SLi setup in the second PCIe x16 slot and have it work correctly?
2. Can I put a non-GPU device (such as a PCIe x4/x8 RAID card) in the second slot and have it work correctly?
Yes, you can support 2 cards for up to 4 monitors. To do that, you have to flip the little card on the board to the "SLI" position. That does NOT enable SLI, but instead splits the pathways for the first slot, to make half of them available to the second slot. SLI is actually a driver setting.
As for putting a smaller card in there, you can do that too. Some boards make the slot x1 when the paddle card is in "non-SLI" position, other boards disable it. Either way, to get more than x1 you have to flip the card to "SLI", which as I said doesn't actually enable SLI, it just sets the pathways to x8.