I am not with Laminar Research: just a flight simmer.
My understanding, having read earlier Q&As in this forum, X-Plane does not work in SLI mode. My memory tells me that Laminar Research does not consider SLI mode as a high priority at this stage. I doubt this has been rectified in the latest Beta version.
Also, the graphics cards have to be identical to be able to have a SLI link. You can check by consulting Dr Google