I don't believe they support sli from what i have read.
I do not see a sli bridge. From what i know to use SLI you need a bridge to connect the cards together. Now most all AMD cards lacking a crossfire bridge communicate through the pci bus. I do not think NV ever implemented that.
2600k HT on@4705mhz 2x EVGA GTX770 4gb Classified cards running in SLI @1320 mhz core and 2003 mhz mem,mounted in CM HAF922.2xHTPC's 2xi3 2120 3.3ghz dual core,1xasus LP HD6570 1xHIS hd7750@1150core1325mem,55"PanyVT30