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Thats near impossible to compare cause every scene is different.
Some scene render much faster on CPU other on GPU and other need a lot of Vram.
Two Titan's would make a lot of hot air in case. Better would be two Quadro Rtx 6000 with blower style coolers... I have to buy one more 2070S because i passed 8 gigs of ram. But you will be pressed hard to use all of 24 gigs from one card...