Quote Originally Posted by DRRosen3 View Post
You only need to level ONE other class to "unlock" advanced jobs.
But since an advanced job is allowed to pull cross-skills from two other classes, it's generally a good idea to level both those classes to get access to all the skills your job could use. Foresight in particular is nice for PLD. (Granted, you don't have to level MRD a whole lot to get it...)