How do you select a second class?

[Tandor]

A job is (literally) a piece of equipment for a given class. When you become a Paladin, you are leveling Gladiator, with the Paladin soul crystal equipped. Whatever level your Gladiator reaches is the level of your Paladin. You don't level them separately.

Conjurer is a separate class. You can level it just like any other class at any time.

Ah thanks, I hadn't realised that. So can you unequip the Paladin soul crystal in order to get the full Gladiator skills and cross-class skills if the situation demands that? Or is a case of once equipped, that's it for ever?