I can confirm unequivocally that, even though X-Plane 11 does not 'officially' support SLI, the settings change as described above by JHO0101 definitely works.
I do not run a very up to date system, with much less performance than stated above (FX-8300 with 2x GTX960s in SLI) and, with everything maxxed out apart from AA I get a solid 30FPS with V-Sync enabled on my old tractor of a system. Previously stuttering was par for the course, now my simming experience is as smooth as silk... Definitely works.
I registered on this forum just to share my opinion - something which I rarely do, but I figured it would help someone else out there not running what would be rated stellar hardware. Give it a go - you might be pleasantly surprised. Not sure about AMD though... But yes, NV SLI definitely makes a huge difference with the right settings enabled in NV control panel.
Just to clarify where to find said settings:
Nvidia control panel > Manage 3D Settings > Global Settings > SLI rendering mode > select Force alternate frame rendering 1
Hope this helps someone! Vanscram out.