This is an issue I have had in the past and just ran into again and almost completely forgot about it untill it was put in another post. So I decided to right up a quick guide. Maybe it will get a sticky, maybe it wont. We see so many great guides get lost, maybe the should make a section for guides and solutions.
This does not guarantee that it will fix your Issues of not being able to enable SLI. Just a possible solution. A possible solution to get your option to enable SLI if it is missing.
How to roll back the Realtek NIC Driver to enable SLI. I have never noticed a difference by doing this in my internet, but if you want SLI, it's either this or search for compatible drivers.
1. Navigate to the control panel and click on Hardware and Sound
2. Click on device Manager and look for Network Adapters.
3. Open the drop down and select the top NIC. Right click and go to properties.
4. Select the Driver Tab at the top and select the option "Roll Back Driver". A reboot of the PC may be necessary.
After all that you should be good to go. Navigate to the Nvidia Control Panel to enable SLI. Happy Gaming.