I realize that there have been several threads similar to this, and I have read through every one that I can find. So far nothing has worked. I have a 660 in PCI x16 slots 1 and 2. They are both firmly in place and connected with an SLI cable. At eye level, everything appears to be intact. The system runs fine, but does not recognize the second card in Device Manager or in the Nvidia Control Panel. I ran the system with a single card, and then swapped to ensure that both cards function properly. I have tried booting without the SLI cable attached, no surprise that this didn't work. I used a driver sweeper to clear off all Nvidia drivers and reinstalled the newest versions. Then I used ASUS EZ Flash 2 to update the mb bios. While I was there, I looked for multi-card display options and did not find anything. After that I went step by step through the P8Z77 manual for setting up dual card SLI. Possibly worth noting... device manager is showing an unknown device under "other devices". I tried installing the 660 drivers for it, but still no luck. I am starting to suspect that the second PCI slot is busted, but I'm not sure how to check since I must have a card in slot 1 to boot. Please help! 
Specs:
CPU: Intel i5-3570 @ 3.4Ghz (OC to ~3.6)
RAM: 2x 4Gb DDR3 ADATA
PSU: Corsair TX850
MB: ASUS P8Z77-V LK
GPU: 2x EVGA Nvidia GeForce GTX 660 2GB
Memory 1: ADATA SP900 64GB SSD (For OS and system stuff)
Memory 2: 1TB (For everything else)
Monitor 1: 23" Asus VH238 through DVI
Monitor 2: 27" Asus VE278Q RT through HDMI
Cooling: Liquid + 3 fans
Specs:
CPU: Intel i5-3570 @ 3.4Ghz (OC to ~3.6)
RAM: 2x 4Gb DDR3 ADATA
PSU: Corsair TX850
MB: ASUS P8Z77-V LK
GPU: 2x EVGA Nvidia GeForce GTX 660 2GB
Memory 1: ADATA SP900 64GB SSD (For OS and system stuff)
Memory 2: 1TB (For everything else)
Monitor 1: 23" Asus VH238 through DVI
Monitor 2: 27" Asus VE278Q RT through HDMI
Cooling: Liquid + 3 fans
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