Since you two are tanks you can get a lot of XP running both Duty Roulette and whatever the highest level dungeon you have available is as tanks usually have fast queue times. Duty roulette has the added benefit of giving you some nice cash rewards if your role is in demand. Compliment this with your storyline quests and you can get to level cap pretty quickly. You can probably get to level cap in 3 weeks to a month playing casually.

You will want to level other classes for abilities as mentioned, but you will also want to do it so that you can unlock your job which provides new abilities and a significant performance/stat boost.