As many have suggested, this would work best as a bonus similar to rested exp. The thing is, for all intents and purposes this is the same system as rested exp. While you're leveling a job, all your rested exp gets used by it, so when that job hits 60 you effectively get more rested exp for other jobs. It's the same effect the OP asked for, only a much simpler system. Well, it would be if you got another static bonus when playing a job other than your highest... which is another system that already exists.

This is trying to solve a problem that's already been solved. If you think you should level alts faster, just ask for bigger bonuses from rested and/or armoury bonuses. No need to add extra complications which would serve only to arbitrarily benefit certain play styles over others.