Leveling Question?

Just wondering what is the best time to start your Duel-Boxing....10 or 20? should i solo level each guy from 1-10 and then start duel-boxing?

I see no reason why you could not start at level 1.

I did all mine at level 1. Helps because you only have one or two abilities, which means you can get used to having your main attack chain on one or two keys and can think about the other important stuff like movement, follow, etc.