My 6GB GTX1060 gets around 22-24 layers of noisy video before it starts to hit around 30fps. Your 8GB GTX1070 should be closer to the 26-28 range. I personally haven't used more than 4-5 clips at once, so I think I'm set for a few years haha. I get about 8-10 on 4k, and my GTX1080 got closer to the 14-16 mark. All of these tests where on laptops keep in mind. Crazy that a mobile machine can push that hard. Exciting times
I wouldn't think having a dual card will help with any processing power, as it's just a single card doing the rendering. But you should get more outputs with that second card. I've never played with 2 GPU's so I'm not sure how stable it would be.