Running with SLI

I have Windows 7 Home Premium x64 and everything runs smoothly.
However, I have two NVIDIA GeForce Go 7950 GTX video cards.
Currently, only one is running.
Here's the problem.
When I try to enable SLI, after the restart, the login screen comes up with a blank, solid black screen.
The Windows logo shows up during the boot process, and when it gets to the point where the login screen should show up, the music still plays.
Once my computer gets into this mode, I can't switch SLI off, and end up needing to run it in safe mode, uninstall the drivers, and then install them again.

I've got the most current drivers, and I'm not sure what to do.
Searching around on the internet has given me no help.

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