There is an EXP boost for crafters/gatherers, but it's not from having another class leveled.

If you go to your GC you can buy (At the first tier) Company Issue Survival / Engineering Manual I and Company Issue Survival / Engineering Manual II.

They cost like 1500 and 2300 respectively and confer a 50% bonus in EXP gained for 2 hours up to a maximum bonus of 20,000 exp and 40,000 exp respectively.

These are best for gathering, but also useful for crafting since there isn't much else to spend GC seals on.

The best way to do it is to buy and use the Survival Manual II, pop it, then buy a 2nd one, and go off to grind on some nodes or Levequests for an hour or so. After level 30 the buff will wear off pretty quickly assuming you're grinding on similar level stuff, so pop the second one once it does. It works out to a bonus 80k exp which isn't super amazing but is pretty good in the grand scheme of things considering how grindy gathering is in general.