Pci express x16 slots not working on asus

Two months ago I have purchased the asus p8z77vlk motherboard that i purchased along with a nvidia gtx660sc graphic card, the motherboard has 3 pcie16 slots #1blue, #2gray & #3black. Fast forward to now i just purchased a second nvidia gtx660sc to run in dual SLI. I bridged the two cards together but i noticed that in the nvidia control panel it shows no SLI connection. Under the device manager display adapters, it shows only one gtx660 card. I've tried updating the card drivers, uninstall, reinstall the cards, I tried each card seperatly to ensure they both work. If you take a single card and connect it into the first pcie 16 slot, the single card works, if you connect a single card into the second PCIe 16 slot the card doesn't work. I even flashed the bios with a new update, i went to the advanced bios check my settings to make sure that my slots were enabled. Why is my #2 gray pcie16 slot not working, should i try to bridge the cars in #1 & #3 slot? what should i do to get SLI, please reply.