Easy way: Power all the risers directly from PSU 1 which is powering motherboard - Card aux power can come from other PSUs.
So this should be true I think.
https://i.imgur.com/BCnc7Uh.jpg
I ended up using one of the USB risers which lets you totally separate the GPU power supply as the only link to the motherboard is data. Plug it and worked first time.
Only down side is there twice the price, But it I had got this from day 1, I would have gained 4 days mining, sound would have paid for itself.
Hope this helps
That is what I would rather do I think. Can you tell me where to get powered USB risers that have only the data lines going to the motherboard? Any other tips are appreciated.