If anything, one way they could force the multi-job per class scenario would be to limit the abilities you can use from the class. The job quests start at 30, so what if they didn't allow you to equip any skills from the class after 30?

The class would have 20 levels of abilities that the Jobs can't use.

Job A would have 20 levels of abilities that define it's purpose.

Job B would have 20 levels of abilities that define & separate it from Job A.