Maybe I am missing it, but I can't find how a Psyker in the main book gets to add a second discipline?

It's under the Psy Rating 'X' Talents on page 120-121 of the core rulebook. Each time you gain a new Psy Rating, from Psy Rating 4 and above, instead of picking Powers from within your current Discipline, you can instead choose a new Discipline, and one Power from that new Discipline.

Another way is to buy the "Psychic Power" talent for 200xp (me thinks). Choose it from another Discipline and you have aquiered a new one.

Gregorius21778 said:

Actually, only if you already have the Discipline. The exact wording of the Psychic Power talent is as follows ...

"You have acquired an additional Psychic Power. You gain one power of any Discipline you know ."

Thus, why all the references to the fact that most Psykers concentrate on one Discipline, since it is much more difficult to master more than one.