Change jobs whenever you like. The game system is built around that ability.

It's actually better to not let yourself get too overlevelled compared to where you are in the story, and you'll probably get more than enough EXP to level two classes at once, especially if you're doing roulettes and/or sidequests as you go.

The game still (incorrectly) tells you that you get a bigger EXP bonus on your second class if there's a bigger level gap between it and your highest class, but this was changed. Now it's a fixed boost for anything that isn't your highest class.