Gtx 1070 and gtx970 can exchange fire.

No, crossfire is the technology of ati (now called amd); N card is sli, and sli needs motherboard support, not all motherboards support it, generally at least Z series motherboards, and sli needs the same type of graphics card. Two different N cards can be used as one of the physical accelerator cards. Physical acceleration is different from sli, which will not greatly improve the performance of graphics cards. It is a waste to use either of these two cards for physical acceleration.