Nah I have a single cable with 2x6+2 and it has 2x 4+4 for 2 motherboards CPU power for some reason.
I have now EVGA GeForce GTX 970 SC with 8pin+6pin power and works ok and I used to have Asus r9 290 so this cable should be fine now that I think about it...
Edit: Now I see that I have another cable with 2x 6+2pins (was hidden XD). So total of 4 PCI-E 8pins.