You learn it at your class trainer, that's the same for every class. Rogues get dual wield at 10, I don't know about the others by heart but just check thottbot for a complete skillist per lvl if you need to know something.
Rogues get it at 10, hunters at 20, warriors at 20 (atleast I'm almost positive - been a while since I played my warrior). No other classes can dual wield. It's taught by your trainer like any other skill.