And you can do this with any class but the ranger (I believe), but since you’re a ninja, that’s the setup. It won’t effect the moveset, but you’ll get all the extra damage and swing speed stats off of your secondary. And also, here’s a tip for ya, from one ninja to another: If you’re rocking the katana (like I like to do,) you can take any one handed weapon (fist or dagger) and middle mouse click it and it will force equip into your second hand slot, even though the katana is a two handed weapon.
And I got my level 13 rogue ninja to level 75. I took an afternoon once and Just walked from sword to sword until I was near a biome border, then I’d cross it, head to town to stock on pots and bottles, and then I’d repeat, just cutting lines through biomes.
As some people have said, walking around to the quests and completing those is the best way to grind out xp.