I have 10 rigs for more than 1 year, some of them are mixing with 3 psus, never had any problems that is power related.
Ps: im using paper clips jumper in all Psus that are not powering motherboard
The rest are just bullshit to sell some gimnicks acessories/psus
#peace
So you're not synchronizing PSUs when powering-on/off? I'm just wondering because I don't do that either and I have up to 4 (four) PSU's on some of my rigs.
One day I will probably measure currents flowing between PCI-e and the risers, but I don't think there is much if anything at all. 3.3V seen on the rizer USB connector is likely signals, not VDD.
PCI-E lanes that carry data are usually AC coupled via small capacitors on all GPUs. They are not isolated (like optically), but not DC coupled either.
-scsi