Besides http://www.tomshardware.com/forum/id-3054416/gtx-970-sli-1070.html which gets into the IDs, the other issue is the rendering seems to run at the slower of the 2 cards.
And benchmarks, reviews at
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-970-vs-Nvidia-GTX-1070/2577vs3609
http://gpuboss.com/gpus/GeForce-GTX-970-vs-GeForce-GTX-1070
I worry the PC PSU would need bump to handle both cards and then well, the 1070 alone would win my use patterns.
I was thinking of upgrading my graphics card from a GTX 970 to a 1070, which seemed like a good idea, since I didn't really want to or could spend $700 on a 1080. I was wondering if it would be possible to set up SLI between my 970 and 1070, or is that just a bunch of ludicrousness?

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