Don't ever mix more than one PSU on a single piece of equipment!

Also, the 8 pin CPU power is 4 ground 4 hot, the 8 pin pcie is 3 hot 5 ground, they are not interchangable.

 It's OK to have three cards and the mobo powered from one and two cards and the HDD powered by the second PSU, but never use two PSU for one card.

12v transformers in a psu put off slightly different outputs and use a ground connection unique to that transformers output. If you give a device two different ground connections then all hell can break loose