2. You said you have three GPUs per PSU but risers on only one? Shouldn't you be matching the 3 GPUs with same 3 connected risers to each PSU? Ideally the risers and boards should each be on the same rail of each PSU or it could cause some issues for any board that would end up being connected to two different rails or worse two separate PSUs. Assuming you don't have it setup this way, are your pigtails on the risers getting warm?
I have my dual PSU setup just like his. For my 5x 280x setup, one PSU powers 3 cards, and the other PSU powers 2 cards, the MB, the HDD, and all 5 risers. Never had an issue.
Someone else was talking about how you can't do it cuz if they cross, you somehow get 24V running through your card? Or MB? I don't remember, but that's BS. 12V is 12V, and the PSUs aren't being run in series in any way.
Someone else was talking about how they can get out of "phase" or "sync", but since ours are running off the same breaker, and are the same brand/make/model, there's no way they could unless one of the PSUs just shits the bed.
TL;DR There's no issue.