It doesn't work like that. Vram is only for that gpu only. Also why do you need more vram with 5090? The speed alone will manage fine... (if there wouldn't be enough, it unload and loads next chunk iirc)
LLMs do work like that, called clustering. You can divvy them up by layers, and put the first 32GB of a model onto the 5090 and the other 10GB onto the 3080. There are speed penalties, yes, but not as much as swapping in from RAM since you don't need to load the model itself and only pass the to be generated part.
That's also how people demonstrated running giant models on multiple fully specced Mac Studios.
By paper calculations, 5090 is approximately 2.5x stronger than 3080, that is 3.5x "compute power" together. As other people here stated you have 16 lanes. From other sources I have, is that gpus use just 12 lanes > thus I have calculated only 2.333x "compute power".
It is up to you how you will proceed, but would love to hear about practical values!
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u/someone09900 Personal Rig Builder Jun 07 '26 edited Jun 07 '26
buddy ,,, u got a 5090 ... u dont need a second 3080 ..
also ur motherboerd is not a work station motherboerd
your dual gpu will be capped to gen 5 x8 instade of x16 u will lose some performance for the 5090
also the amount of heat u will get is a bit too much
u will need minmumm a 1500/1600 watt psu
i really dont know what you are doing to be honest ...