In most cases, you don't have to do anything except plug in everything correctly - i.e. most dual-rail PS units dedicate one rail to the mobo / cpu connectors, and the other rail supplies power to the other connectors like hdd/pcie/floppy/sata.
if you have a SLI motherboard (I am not suggesting nor recommending an sli config just a mobo example) there is an "EZ plug" that fits a 4-pin HDD power input to supply extra power to the PCIE bus, and the PCIE 6-pin connector itself to the GPU those are a few ways the 12v power is "shared" and not to mention the power supply itself may have the ability to load-balance the current demand between the rails.