Job quest class requirements

[Taika]

You'll only need to have one of the subclasses at level 15 to unlock the job (the one mentioned in the class level requirement list). How high you'll want to level the subclasses depends on which abilities you want to use from them (after taking a quick glance on class abilities the highest cross-classable abilities for every class seem to be level 34, so that's where you're likely to level both of your sub-classes).