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 sortNot at all, especially if you are using good quality risers.
I've always used USB risers, never had any beef with those wonky IDE-like risers