This wasn't a ground loop issue, both grounds are connected to house grounds, so were almost certainly at the same level, and would equalize potential when plugged in, not when powered. What happened was that the power rails on one psu had a different potential than the other, think like 11.5v vs 12.5v is technically possible. 1v Across solid copper traces and wires would go through the mobo and GPU when powered.
What happens here is that one power supply 'wins', that is the one with the higher voltage, and it provides current to the second. PC Power supplies are not designed to accept or 'sink' current, and are destroyed quickly.
When using dual PSUs, you can only use them with GPU risers to isolate the power from the motherboard, or with sata sds with no power connections. A m2 port or pcie port has 12v power traces.
This wasn't a ground loop issue, both grounds are connected to house grounds, so were almost certainly at the same level, and would equalize potential when plugged in, not when powered. What happened was that the power rails on one psu had a different potential than the other, think like 11.5v vs 12.5v is technically possible. 1v Across solid copper traces and wires would go through the mobo and GPU when powered.
What happens here is that one power supply 'wins', that is the one with the higher voltage, and it provides current to the second. PC Power supplies are not designed to accept or 'sink' current, and are destroyed quickly.
When using dual PSUs, you can only use them with GPU risers to isolate the power from the motherboard, or with sata sds with no power connections. A m2 port or pcie port has 12v power traces.