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You'll have to force Alternate Frame Rendering to 2 in your Nvidia Control Panel to get SLi working. You can learn more about doing that and read about people's experiences with it https://www.reddit.com/r/fo4/comments/3s5ldy/how_to_get_sli_working_in_fallout_4/

Still doesnt work i still get 39 fps on Medium with 2 960's

Question of my own here.

I thought SLI was strictly hardware and device driver across two GPUs, and had nothing to do with the applications (like Fallout 4) that use it.

Are games even aware that SLI is being used?

You'd get higher with just 1. SLI is such a headache. It has been for years, I honestly don't know why people bother.

I thought SLI was strictly hardware and device driver across two GPUs, and had nothing to do with the applications (like Fallout 4) that use it.

That's not quite correct!

Every game needs a proper profile to be created in addition to some work in the games engine to take advantage of multiple GPU’s.

So, both NVIDIA or Ati and the game developer must work together to optimize this profile and the engine, so that it actually scales and works well.

Theoretically every graphics engine can make use of SLI. That doesn’t mean it’s an easy task, but its technically possible.

Also a lot of work must be done in order to optimize physics data that shares frames beetwen a SLI configuration.