Read about it here plus countless posts on Reddit..:
https://www.nicehash.com/blog/post/mining-psu-power-supply-buying-guide "Note that you must power GPU and USB riser connected to the GPU from the same PSU or risk damaging the components."
https://www.thegeekpub.com/11955/using-dual-power-supplies-mining/ "On power supply two, make sure the VGA power connector and riser power both come form the same power supply. Never plug the riser into power supply one, and the GPU into power supply two."
https://2miners.com/blog/build-ethereum-mining-rig-2019/. "Important: make sure you always plug the cables in so that the GPU and its riser are all powered from the same PSU!"
Another posts just wrights "don`t do it!"
So, if you have different outlets with different phasing it can become a problem, i agree. But i haven`t ever saw such silly decision from miners.