So the 4 pin to 4 pin cable on the left is supplying ground and 12 volts to both power supplies. So apparently immediately prior to this part of the circuit, the board must step down the incoming 24v power.
The 4pin to 4 pin cable on the left are supplying GND and 20V in may case (can be 18 to 28V) depends on your monitor but the color in the schematic is not Yellow as for the 12V is a different color closer to bronze and that represent Vin that can be 18V to 28V.
The biggest coil on the board is responsible for converting the input voltage Vin (18 to 28V) in to 12V.
The 20 pin cable supplies:
1. seven ground wires (if the 4 pin cable on the left is supplying two grounds, why are you bothering to supply seven ground wires on the 20 pin cable)
2. One Power On Wire (I guess I understand that, earlier you said even a few ms differential between the two causes some kind of shut down problem)
3. One 5 volt wire (if the first board can convert 24v ti 5 v, why can't the second board take the 12 volts it is getting from the 4 pin cable on the left and convert it to 5v?)
Can you answer those these questions or comment on what I think I see happening?
1.More GND connexion is better do not want to enter in details and is easier for me not to remove them
2.That was a different problem now the green wire is necessary to power ON and OFF the PSUs
3.Look closer at the drawing and photo the 5V and 12V wire from the 20 pin ATX conector + two GND wire are used to supply the HDDs and ODD see that this wires are just going to an SATA power cable and a 4 pin Molex power cable.
There is no connexion from one board to the other for 12V or 5V just zoom the photo to see better.
Half of energy goes to HDD+CPU, another half to MB -- yeah, that's a good balanced scheme. You can insert into MB things like ATI Radeon 5670. Nice twinning, electrodacus!
Thanks Ksanderash you can use any graphic card that dose not use an external power probably 5670 is one of the best right now.