electricity moves from high voltage to low voltage. it doesnt move 12v -> 12v. It moves 12v -> 0v (ground)
This caught my attention, though I know what you are trying to convey, the actual statement is so wrong.
Voltage does not move. You cannot have a 12vdc line connected to a 5vdc line and have the voltage "move". Nor can you have a 12vdc line connected straight to ground without burning the line, this is called a "short circuit".
Current flows. If flows from negative to positive.
The voltage is the pressure for the current.
To get current to flow, you need circuit. A circuit is a complete "loop" or "circle" that includes an energy source, a load, and ground.
Lets use a fan in our circuit as the load device. When we turn the fan on, energy or "power" is expended as heat. This is measured in watts.
So we have:
current which is amps using the symbol "I"
voltage using the symbol "E"
watts using the symbol "P"
If you remember "PIE", you can figure the number for current, voltage, or watts with this formula:
P over I times E
P I * E
P=I * E
I= P / E
E= P / I
To the posts leading up to the above statement, someone already said what was needed for multi-psu systems, and that is a "common ground".
The PSU's, MB, GPU's all need to be grounded to the same source. That is normally accomplished when everything is screwed into a case. The metal tabs that hold the GPU's to the case are grounded on the PCB of the GPU card. The PSU is screwed down to the case and the MB is properly grounded when screwed onto the backing plate through the brass/steel risers.
The same "common grounding" needs to be accomplished with our open-air builds especially with multi-PSU's builds. The current in the circuit flows "everywhere" but will take the path of least resistance so having a common ground is important.
For those using the squared aluminum pieces to make your open-aired frames, if you are using plastic connectors in the tubes, you will need to ground each segment.
For the wood based frames, using strips of metal connected with 12-gauge wire to connect all grounds together will work.
For milk carton cases, build a common ground to all components with 12-gauge wire.
By creating the common ground, you will prevent most of the problems with wires burning up or getting to hot that some have already experienced.