I was recently trying to explain to someone how the PowerCooler psu they bought wasn't very good when I came up with a question of my own.
I think I understand multi 12V rails because I remember using them years ago. This psu had 2 12V rails. The first had 20A max and the 2nd had 17A max with a total of 340W (only 28.3A max). My question is the 20/24 pin included as one of the rails? If so I suspect it is rail 1?
"Multi-Rail" is nothing more than a marketing gimmick. Good single rail PSUs and good multi rail PSUs trade blows all the time. Some of the best PSUs use multiple internal regulators and have their combined output connected to form a single rail.