I had SLI 2080 Ti for a bit. Too many problems, so I sold the second card. Surprisingly, performance is about the same, even in games where SLI looked like it was working. A few games did take advantage of the cards, like Tomb Raider, because they support DX12 mGPU. And GTA V and a few random games did have good scaling. But overall most games either did not work, or "worked" but without substantial performance improvements.