The amps don't have to come from a single rail ( and won't ), they reccomend that your psu have a total 12v output of 30a.
If it will make you more comfortable, the actual OCP trip point is 40ahttp://www.antec.com/Believe_it/product.php?id=MTc1OQ==
Nothing to worry about.
If they recommend that the PSU to have a total 12v output of 30a, are you implying that the amps are additive?
I thought you can't add amps coming from other rails so that it can exceed 30amps?