While I understand your desire, there's one reason why this likely won't happen: Money.

Let's just say, for ease, that it takes 2 months to fully gear one class. You're paying for two months of subscription. That amount wouldn't change if you could gear another class at the same time, it would still be 2 months.

But if you follow traditional MMO gating, suddenly you have 4 months of subscription to gear two classes. Logically, this makes a lot more sense for SE to do than to allow you to gear multiple classes at once.