A PSU with 'dual' 12v rails effectively has all its 12v components 'doubled up' to give more total Ampage.
There are alot of technical issues with making one massive rated 12v rail, its very complex to make a system that will provide stable, reliable voltage of 12v with no changes due to differing loadings from 0.1A all the way to say, 50A.
Its alot easier to make a system that provides from 0.1A to 25A twice on two seperate rails, hence why we see Dual rails.
Some extremely high rated PSUs use 3 rails. My 680w model does (14A, 15A, and 23A) and the PCP&C 800w and 1000w PSUs do.