Most people I've spoken to multibox because they enjoy it. Pick a composition that you think you'll enjoy playing or will be successful in other aspects of the game like raiding or PvP.

If your goal is just to farm more efficiently then you're probably better off simply levelling a mage with mining and herbalism and following one of the Mara/BRD/DM farm guides on YouTube that can earn you 150+ gold per hour plus herbs, mining nodes, etc.

https://www.youtube.com/watch?v=9bT2vq81rSQ