This may be a dumb question but is it possible to merge two +12v rails in a PSU? Specifcaly I have FSP550-60PLN

One rail is deticated to the 8pin motherboard power connector and the other rail is everything else.. the 24pin header and all the I/O device ports. I'm loading up the I/O side I think too much.

Second question.. If I put my Kill-A-Watt meter on just my PC measure the watts used, then take the average effiency of that unit (which at full load is 62% ) so say 68% at mid load?, and multiply it by that.. Would that give me the combined wattage of my machine (all rails and voltages considerd)?

Hope that is understandable as work is pulling me away now