I think I just looked up that PSU and with a single rail at 54A for the +12V this goes to, there isn't much concern about the PSU.
Remember what you do is YOUR CHOICE.
I wouldn't because the GPUs can yank a lot of power and there is some drop on the wire. To avoid this I'd use two connections with some adapter to tap some 4 pin Molex to PCIe power or even the extra SATA power connection to PCIe power.
There's another issue that only you can answer. Does this cable reach to both cards?
Finally there are SCREAMS as folk mix GPUs and want to kill others about how hard to impossible it is to get them playing together.
Hello I have the Corsair RM650 PSU and I got cables which are split in two ends (6+2 pins). Can I just use one cable to power my GTX 1070 and RX 570 at once?