I have 2 psu rig, and working fine. Try to powering 3 risers and 3 cards with first PSU, and 3:3 with second. Don't cross riser and cards with both PSU's.
Sorry for my bad english, i hope you can understand me.
I made sure not to cross riser and cards. My secondary PSU is only powering the graphics cards, not the risers. I have two cards connected to the motherboard with no powered risers, and two cards connected to powered risers powered by the primary PSU. I'm not connecting my secondary PSU to the two powered risers because of this warning from BitsBeTrippin' on youtube (
https://www.youtube.com/watch?v=xZiWciJLK3o):
NOTE: Ensure one PSU is used for the Motherboard and ALL connections (molexs/sata power connections/ & any other non-GPU connections) The Second PSU should be ONLY used for powering the extra graphics cards ONLY, just the 12v rail connections.
DO NOT USE THE SECOND PSU to power any of the risers...this will create a weird loop back through the MB and potentially nuke it.
Which PSUs (brand + model) are you using for your setup? Thanks.