You don't need any of that.
Just buy 2 PSUs. Make sure they are grounded together. Use a paperclip for the aux PSU.
Start the AUX PSU first and 0.5 seconds later start the main PSU.
Been working since 2013.
I have to ask you - since this has been the issue that I've argued endlessly with people about - do you only use the PCIe outputs on the second PSU, and power ALL risers from the one that powers the motherboard? This was what I was taught by an electronics engineer, but when you get into mobos that can have 13 GPUs on them, where the hell will you find a PSU that can power 13 risers? I guess you just modify and extend molex outputs....
It doesn't matter which is plugged in where.
Just make sure you don't mix the +12V between the 2 PSUs. So if a GPU requires 2 x 6 pin connector, connect both of those connectors to 1 PSU only. Because they are shorted together on the GPU and if you connect from 1 of each, you will damage your PSUs.