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The last time I tried to use two PSUs on one mobo. I killed one of the PSU. So not a good idea to use 2 on the same board. I think you need to buy ADD2PSU. They are only for sale in the US.
I also use Corsair. But a lot of people are in the Seasonic camp.
I was planning to order Add2PSU from their website. I'll need to check if they have issues shipping internationally.
I was planning to power the system and 3 GPU with one 860W PSU. The other 3 cards, and their powered risers will be connected to the other 860W PSU.
Do you think this configuration will be fine?