I use 1 PSU for the board and all risers, the second just to power cards
^^^ This is a perfect example of how to start a fire either by crossing voltages or one psu crapping out while the other is still on.
*NEVER* power the riser and gpu from different psu's.
*ALWAYS* power the riser and gpu from the same psu.
Nothing will happen because there is no potential differences, and however if there will occur a lack of power on some components, system will shut itself down immediately... Also nothing will happen if you lose powering on one of PSU - it will cause the same.
I had many times back to my mining rigs to see that the motherboard is not powered due to PSU failure but the orher one is running still powering risers or GPU's external input or conversely - the cards were not powered but the mobo (system) was running with black screen, but after reboot it was everything OK anyway.
Guys, come on these are not 80's electronics, I know it is chinese crap done for $10 but we still have some protective circuit inside of it.
Regards