Something Productive to do while Leveling my 2nd Job?

[Ehr]

Hm, there honestly isn't much you can do that would yield a reward to your higher level job from leveling a lower level one (other than the actual leveling itself and getting access to cross class abilities). Just use FATEs, Hunting Log, and Levequests to get Marauder to 15. Also, if you haven't, you should try to get Pugilist to whatever level Internal Release is (it's sub-15 at least, shouldn't be too hard) so you can Cross Class it onto your Dragoon.

Though, I suppose something you can consider productive (and what I did for leveling DPS classes) is to just DF dungeons for leveling once you hit 15 and go level a crafter or gatherer. But that's only really something I did when leveling my DPS classes after I had finished the Main Scenario Questline.