How do I connect power to a NVIDIA GTX 1070 Founders Edition in a 2009 Mac Pro?
You will need a Y cable with 2 x male mini 6 pins at one end and 1 x male 8 pin on the other end.

You will have to connect both mini 6 pin ends to the motherboard. 1 x 6 pin will not provide enough power. Only both 6 pin ends will be able to provide the power your GPU needs under full stress.