The subjob is half at MAX. You still have to level up the other job. If you have a 20 warrior and a 20 monk and decide to make either your primary job, your sub job will only be level 10 for either. If you have a 20 warrior and an 8 monk, and warrior is primary then you will have a 20 warrior / 8 monk, whereas if monk is primary then you have an 8 monk, 4 warrior. Hope this makes sense.

Leighton