I don't know what you've done in the installation process, but I've managed to get HyperSLI running in Windows 7 Ulimate 64 bit and in Windows 8 Pro 64 bit.
MOBO: Gigabyte GA-970a-DS3
GPUS: EVGA Nvidia GTX 550 ti FPB [COMPLETELY IDENTICAL CARDS]
For Windows 7, after you've completed the installation process and have restarted you computer, you may have to run msconfig and choose to load Windows 7 HyperSLI on startup, then restart once more. You should then get the option in Nvidia Control Panel to enable SLI. I'm not sure if this differs from motherboard to motherboard though. I wouldn't think your motherboard is the problem since you've managed to get it running in Windows 8. Another thing I was thinking about is, if you have multiple HDD's and/or SSD's, make sure you have installed HyperSLI on the same disk drive as your OS.
Just a thought for you to try if you'd rather use Windows 7. I've actually come to terms with Windows 8. I used Start8 software by Stardock to bring back the start menu and it rocks! I have Win 8 with HyperSLI installed on a SSD and it boots up lightning fast compared to Win 7 with HyperSLI on a standard HDD.
Edited by Pimphare - 8/22/13 at 4:12pm