well, the training of the mortals may very well be just a part of the chain of events that needs to transpire in order for the tail bit of the prophecy to be fulfilled. prophecies, i think are just stimulus. when people believe that they know who/what the subject of a prophechy is, for example, it will lead them to an event that will trigger a domino effect. in this prophecy's case the taking of the book.
for centuries, Nicholas has been convinced that the two that are one are twins. and not just any twins, but that ressembling the Original Twins auras/power. he didn't think that Clarent and Excalubir could be what the prophecy fortells. also, Dee have always wanted the Codex for his master and pursued it for centuries too. so these two separate events in themselves will eventually lead (or have already led) to the first part of the prophecy -"when the Book is taken"
if we think about it, it could have stopped there had Josh and Sophie not believed Nick and followed him to Scatty's dojo. which then, might not have resulted to Dee asking the Morrigan's assistance (assuming Dee is the "Queen's man" in question).
Basically, it all boils down to choices and decisions made. like i said domino effect. whether or not Dee is the queen's man in the prophecy, and Machiavelli (if he's the Q'sM)dosen't choose to ally himself w/ the Crow, at this point, it maybe a moot point becuase the mortals have already been trained in some magics. so it can only mean that the two that are one must become the one that is all.
i do apologize for the lengthy pitch. it's just my take on prophecies and fortune tellers in general. and as we all know, in this series, there are no coinsidences...meaning what i just explained is incorrect and i wasted of your time
that's just 1 way of looking at the prophecy. we have yet to discover if most of its contetns have already played out