My colleagues have been running Rx 580 Sapphire with 2 PSU-s x 750 watt each Modular from the Cooler Master brand. They all have connected the risers with Sata cables and they have been running from about 2 months and a half without problems.
They are using risers version 009s which I recommend you do also.
That's similar to what I had, two PSUs with a Y adaptor to the motherboard. Then RX580s on risers with Sata adaptors.
I too thought 'everything was fine' and ran 4 months almost continuously with minimal problems. I thought I wouldn't have issues but then here I am with the costs of replacing the PSU and a riser and the fact the rig did - in essence - fume while unattended.
I bet if your colleagues touch the left hand side of the plastic sata adaptor it will be very hot, not a good sign.