Just a thought. If you have been playing the game since 03 - 04, then you probably have already played all the jobs already and have 1) a pretty decked out melee class job and 2) a seriously pimped Mage class to alternate for end game events as needed. That being said, I cannot imagine a serious player not leveling both to see how they can better enhance their favorite jobs with a new combination sub job.
Think about how ninja changed all melee play. Or how dancer as a sub enabled many people to solo abyssea and leveling a new job in the old areas. How about the revolution of /sch to just about every mage job that truly was the nail in a coffin for RDM.
This being said. I do belive that you will see alot of people leveling both jobs in the mines/chests/abyssea as fast as possible so they can go xplore in the new areas of SOA so they can see how the mechanics of both work as a main and a sub job. Since leveling time from 1- 99 is < a week for even casual players I am pretty certain this is how it will go. Lets just hope the quests for both arent really stupid in that you need a r/x drop item that only can be had in 1 area each or we'll have massive congestion similar to the mini expansion fiasco all over again. :(