Do you prefer close quarters or ranged? Do you want to level fast to more quickly get your dual class skills, or slow? Do you prefer being a part of the action or just support? Do you have enough adena to invest in classes like mages that require you to buy entirely different setups from dyes, jewels, cloaks, circlet, armor, weapon, etc., or would you prefer to keep it simple and use most of the same equipment for both your main and dual? Once you answer those questions, you'll have a good idea of what you want to play as your dual.