Scratch my post, that combo wasn’t working either, here’s that combo:
https://forums.developer.nvidia.com/t/using-an-a6000-2-slot-nvlink-bracket-on-geforce-3090/169118
So while this company claims to have tested 3090’s + A6000 2-slot bridge on Windows and found it working:
https://www.pugetsystems.com/labs/articles/NVIDIA-NVLink-2021-Update-and-Compatibility-Chart-2074/
Nobody seems to have managed to make it work with any of the bridges (2,3,4-slot A6000,3090), neither on Linux nor on Windows.
https://forums.developer.nvidia.com/t/making-work-nvlink-on-windows-10-with-dual-3090/175086
regardless of SLI-capable board or not.
OTOH, nvlink on A6000 just works plug-and-play:
https://forums.developer.nvidia.com/t/two-rtx-a6000-nvlink-link-is-supported-false/177285/4