Im assuming there are specific motherboards by now that endorse SLI for this setup? Either that or you can get varying sizes of bridge to suit the distance apart they are? Either way, I thought they done away with the ribbon/bridge last gen or two meaning the link was achieved in the PCI level but obviously Im wrong on that as linking two 3090's seems to be hard judging by the many popular channels struggling when it was launched back in October time.
You can get 3 slot and 2 slot spacing now but they are for the A6000 card and work on 3090s too, with of course the official 4 slot 3090 nvlink too, but so far only one x570 motherboard that does 4 slot spacing and that's the MSI GodLike x570 if you want to use a AMD 5000 series cpu.
NVIDIA NVLink 2021 Update and Compatibility Chart
https://www.pugetsystems.com/labs/articles/NVIDIA-NVLink-2021-Update-and-Compatibility-Chart-2074/
Basically the A6000 3 slot and 2 slot NVlink bridges will work on 3090s in SLI too. So now you can SLI without a 4 slot space motherboard with the A6000 bridges. (but make sure you have good air flow if not using blower 3090's or water cooled). 4-slot spacing is fine with air cooling FE or AIBs that are none blower, but make sure case has good air flow).
NVIDIA GeForce RTX 3090 4-slot and RTX A6000 2-slot NVLink Bridges
My setup below is basically that a MSI Godlike X570 with two 3090s in. :-
https://www.overclockers.co.uk/foru...-owners-thread.18898762/page-48#post-34846430
Also this new x570 from asus coming out soon looks like it will do 4 slot spacing too so the 4 slot bridge for the 3090 will work and really if doing air setup you want the 4 slot option and 3 and 2 should be water cooled cards only.
ROG CROSSHAIR VIII EXTREME
https://rog.asus.com/motherboards/rog-crosshair/rog-crosshair-viii-extreme-model/
Other options will have to be x299 or the threadripper boards with the right 4 slot spacing if you want to use the air coolers on the cards and the 4 slot 3090 bridge. 3 slot would have worked fine with the blower cards even the 2 slot, but as we know Nvidia nuked the AIBS from making 3090 blower cards.
Most important thing is to make sure to have 2 of the same card as the NVLINK fingers will not line up as they are all spaced differently and even different height cards too so one maybe taller than the other and the nvlinks are solid and will not connect, no more flexible ones as these are highspeed links.
The popular channels made a drama out of nothing really, you just need to make sure the motherboard has 4 slot spacing or the right slot spacing you need if watercooling or using blower cards and making sure to use same cards and all works no problems. Not had a single problem with mine, it just works as Mr Nvidia says..
The channels were using different cards and had to use a riser to get the nvlink fingers to line up and use the bridge. You are thinking of gamers nexus I think, linus did it right first time with same cards and a 4 slot spacing motherboard so no risers needed.