Speed comparisons between generations requires people to actually have the cards to test. Puget Systems has already done benchmarks with the 4090 and Blender, so that helps for those that don't have 5+ grand to waste, just to find out. The 4090 is over just over twice as fast as a the flagship 3rd gen 3090 ti. So no, two 3080 ti's would never be twice as fast. 

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