I could actually see, for many of the classes, a sort of "exchange program" for jobs.

Conjurers would leave the woods of Gridania to visit the Milvaneth Sacrarium in Ul'dah, to learn more about life and divine powers. Using both the magic of nature and the wisdom of the Twelve, the White Mage would become a beacon of light and hope for all of Eorzea in these dark times.

Likewise, Thaumaturges would head to Gridania, witnessing the sheer power of the Elementals and wishing to harness such power for themselves. With the power of Thaumaturgy and the might of Elemental magic at their disposal, all foes shall flee in terror at the unbridled power of the Black Mage!

I dunno. It's how I always figured it. Thaumaturges did always seem like the class more willing to go to extreme lengths for power, especially if that power was really, really, REALLY needed, RIGHT NOW, like if the Empire marching on Ul'dah. Likewise Conjurers are naturally-gifted with powers, but their sheltered lifestyle could lead the Gridanian-based guild to seek out alternate sources of power -- but nothing as dark as Thaumaturgy, thus they would turn to the Sacrarium.