This is how I see it now.
With their example of how to get a paladin you need Gladiator at 30, and marauder at 15.

If you we take two classes to get a job, then this is what might happen.
Black Mage needs 30 Thm and 15 Cjn.

White Mage needs Cjn 30 and Thm 15

Red Mage needs Blm/Whm 30?