If you choose to use 2 PSU's then you need to make damn good and sure that you select a single PSU to power up both the riser and GPU. DO NOT mix the power sources on risers and GPU's or you can cause damage to your equipment and possibly start a fire!
So true !
https://www.thegeekpub.com/11955/using-dual-power-supplies-mining/
The key is :
Everything you plug on the MB comes from one PSU (24 pin, CPU pin, molex etc ...)
Everything you plug for a GPU comes from one PSU (Riser + 6/8 pins PCI-E connector)
So you will not mix the ground of the 2 PSU and everything will be fine