It doesn't support sli because it doesn't have 2 pci-e slots for graphics cards to go into. Being the GTX 690 is two graphics cards on one pcb (backplate of the graphics card) you should be able to use it in your motherboard just fine. Just make sure you have a sufficient power supply and connectors. Hope this helped!
Agreed it will work this is a perfect solution for someone who doesn't have the ability to place 2 cards in there case. You will be fine powering up your 690 given you have a power supply that can support it.
PSU and case will be the real possible constraints. The 690 is the lowest-power dual-gpu card ever, but I'd still recommend a quality 600w PSU for it. And the 690 won't fit in most mid tower case designs, just something to keep in mind.
You can run two gtx 690's you would just need a tower that could support it but either way once you get beyond 3 way sli things tend to hurt performance in games rather then help. Plus his motherboard won't support SLi anyway well slot wise at least
I already purchased the GTX 690, should be coming in on the ninth. As for the case and power supply, I'm putting the whole thing into an NZXT Vulcan running with a SeaSonic X-850. Nit planning on buying a second GPU for another 4-5 years.