Basically we have 2 groups of people
Group 1 - Power MBO, HDD + all riser from one PSU and all the cards from other PSU
Group 2 - Power MBO , HDD from one PSU and all the cards with their riser from other PSU
I Don't know which one is correct, because all options will work until you don't get difference in electric current then it will all go to smoke.
There is no harm in first case at all.
But. Ya can load first PSU with some GPU in addition,
The point was: power supply of MB and all risers should came from the same PSU