On the dual rail, you’ll probably have to crack it open and trace the wires to see what connects to what rail. You cna also google the exact model number to see if any PC site has done a review which might have the details.
With one rail, you can parallel however many connectors you want, or at least wires - the 20 pin has multiple wires for 12V so you can parallel all those for higher current. You can either prallel everything, then divide this up by fuses to individual runs, or parallel 2-4 wires for each 5-6 amp run - again with a fuse or breaker on each one. Either way is electrically equivalent.
Seems that one rail goes to a 4-pin connector, which can take 8A across each pin - 16A in total. I’ll use this to drive the 6A circuit and one of the 4A, then the other 12A from the other rail.
This is why I wire mine into a breakout box. Inside the breakout box I take the PC board connector and 2 PCIe connectors and wire the +12V together inside the box and crimp then together with a spade and then solder that to be double sure. I then use a single ground of 12 gauge which are spaded to each terminal.