Google Translation from Japanese Lodestone:
Currently displayed as "FINAL FANTASY XIV Realm Reborn" in the item "program to customize" You have graphics drivers that are exposed from NVIDIA Corporation Regarding "GeForce 320.49 Driver Beta", "NVIDIA Control Panel" → to "Managing 3D Settings" will be.

However, in the newborn β3 version FFXIV, SLI support of the game program side does not yet completed. Towards the official launch, we are promoting the SLI-ready, but we ask that you can wait until a formal response completion
Or in other words, SLI is not ready yet even if it is listed on the drivers and to wait for official support confirmation.