How can there be a White Mage and a Black Mage simultaneously on a character?

[AspectOfWinter]

Because you are the "one" and it has no baring on the individual story lines. I have power over the void, nature, my inner dragon, my inner chakra, ancient Nymian war tactics, the power to reshape aether, my inner fury, the will to protect others, and can carry a tune while retelling tales of ages past. Oh yeah I'm also the master of all types of crafting known to Eorzean. What of it? Lol.

If that's the case, then I really think the dialogues in some of the stories could've been worded differently. From what the Padjal in the White Mage storyline says to you, it's almost as though taking up a Job class is effectively the same thing as joining a Grand Company.

[Wolfandre]

It's not quite the case. We can change jobs because SE allows us to, not because we happen to be the one guy in existence who is great at everything. Think of the stories you unravel as mutually exclusive to you. Meaning, in that moment, you are no longer the promising Thaumaturge who is destined to close the Voidgate. You never were. You are instead the promising Conjurer who is tasked with helping Raya-O-Senn in her ritual. It's hardly believable that you are not only the only non-Padjal White Mage, but also the chosen Black Mage to seal the Voidgate, and the adventurer who single-handedly brought the Empire to its knees, ad infinitum.

This is just a story-telling mechanic in order to allow us to experience the content they created for us.

[VisRalis]

If it helps, think of it as perhaps canonically you finish the story with 1 job.
Which is the usual case anyways right?

Regarding red mage unlocked from white & black... well good point.
It'd make sense if its CNJ & THM though.

[PArcher]

It's not quite the case. We can change jobs because SE allows us to, not because we happen to be the one guy in existence who is great at everything. Think of the stories you unravel as mutually exclusive to you. Meaning, in that moment, you are no longer the promising Thaumaturge who is destined to close the Voidgate. You never were. You are instead the promising Conjurer who is tasked with helping Raya-O-Senn in her ritual. It's hardly believable that you are not only the only non-Padjal White Mage, but also the chosen Black Mage to seal the Voidgate, and the adventurer who single-handedly brought the Empire to its knees, ad infinitum.

This is just a story-telling mechanic in order to allow us to experience the content they created for us.

Yeah, I'm going with this. Lore-wise, you are ONLY the class you currently are.

Its just a gameplay mechanic.

[Skies]

If that's the case, then I really think the dialogues in some of the stories could've been worded differently. From what the Padjal in the White Mage storyline says to you, it's almost as though taking up a Job class is effectively the same thing as joining a Grand Company.

It would be, were you actually a Padjal, you are a very particular exception as you are deemed specially worthy and kind of nescessary.

And on that note you need to remember that White and Black Magic are not actually too different! After all it WAS White Magic that nearly destroyed the world once. Both schools of magic were created around the same era and unless my memory is far too foggy they possess the same base casting method, difference being that White Magic draws forth magic from the environment. Hell, unlike Conjury, White Magic doesn't even need permission to draw that energy, you just take, the reason why it brought an Umbral Era to begin with.