it may actually go the other way, needing multiple classes to unlock a single job.

from this interview at rpgsite: http://www.rpgamer.com/news/Q4-2011/101811b.html

the relevant bit is this:

Another major change in the pipeline is the addition of a job system to compliment the current class system. Yoshida states that jobs will not be replacing the classes, but will be built upon them when they are introduced in version 1.21, coming in early 2012. The example given was for unlocking the Paladin job quest. Players would first need to level the Gladiator and Marauder classes to level thirty and then complete a job quest. Once completed, Paladin would become available on top of the class system, almost like a subjob. Leveling up the Paladin would also increase the strength of the original job, Gladiator, and open a new job-specific quest every five levels or so. These jobs will grant unique skills and bonuses that can mix and match with the original armory class system, so a Paladin would gain abilities to help in playing a tank role.
so we might have to level multiple classes that all contribute to the skills of that job.

i can easily imagine needing gla,con and thm to unlock red mage, since red mage has features of all those classes.