Yes you can connect 2 different PSUs using ADD2PSU. That's what it was made for. Been using one for many months now.
Thanks a lot, that's what I was looking for
I repeat, the 2nd PSU will be powering 3 cards along with their risers separately from everything else, although all 6 GPUs will be connected to the same motherboard. There shouldn't be a risk of mixing electricity or anything of the sort