the problem is not the amount of current, it's mixing grounds from multiple sources
actually grounds from multiple sources SHOULD be connected to have the same potential among power sources.
powering USB risers from secondary PSU is OK and works on my farm
I would bet BBT has built more rigs than most here. I would take his advice and that of a PSU manufacturer that you should never mix power connections to the motherboard, which is exactly what's happening when you power a riser with another PSU that's not connected to the motherboard.
you were told already, when using USB risers you do not send any power to motherboard, dunno what it is so hard for you to comprehend...
btw that BBT idiot claimed sgminer in linux do not have RX hashrate drop, guess what? it have...