For your first class, I recommend just taking it up to 50 more or less on its own (Get the other class you need at 15 for your job, but that's it). Should be able to do that easily enough using quests (Keep in mind that going through the main quest unlocks more sidequests in the area the main quest is at) for xp and gear. When you level your other stuff you won't have quests, so that means running dungeons for xp, and crafting gear (I feel dungeons don't drop suitable loot often enough to stay well geared on dungeon loot alone). So, level your crafters and gathers alongside your new classes.