Nothing but bad advice in this thread.
You should NEVER connect any GPUs (unless its the older 7970/280x/290) on any SATA powered cable for the riser.
Since these GPUs take less power from the PCIe connectors (from PSU) they pull more power from the Riser +12V. Most RX series cards use about 40-50 Watts, which is 4 AMPS which the SATA connector cannot handle.
If you have a R9 280X, then you can use the riser SATA.
I believe you are mistaken, my fellow bitcointalker.
SATA = 3 pins rated @ 1.5A each, so max 4.5A in total.
At 12V this equates to 54W which is perfectly fine.
So 1 riser = perfectly safe. 2 risers, well, it's above spec, but many people still consider it as ok (as long as ambient temp isn't too high). Anything above and you're seriously asking for trouble.