When using a dual power supply, you cannot use the voltage sides of the power supply, such as -12v, +5v, +3.3V. In this case, it means that you are not really using the power supply that you purchased at the apparent power. However, you use the + 12v line and the power it gives you. When buying a power supply, also pay attention to how many outputs the +12v side has. If there are two separate lines, the system will not work decisively if you do not distribute them evenly between the GPUs.
You mean power supplies with multiple rails? This is why I prefer PSUs with one huge single 12V rail instead of those with multiple 12V rails. Server supplies are good because they usually have just one beefy rail.
At the very least, the number of rails needs to match the number of video cards. Let's say I have a 1400w PSU with 4 12v rails @ 30A each. I can either power one RTX 3080 Ti on each rail (4 cards total), or two RTX 3060's on each rail (8 cards total). I would not use RTX 3070's unless they are downclocked to RTX 3060 levels.
I'm not worried about this problem with ASICs, since I just use Bitmain or server PSUs.