Why do drills with one ball, when you can do them with two balls? Dribbling with two balls is a quick way to improve your dribbling skills. Plus, using two balls will get you used to doing two things at once when dribbling. The will come into play during game time when you need to be aware of where defenders are or are looking for the open player on a fast break.
Dribbling with two balls goes with the theme of getting better faster. Dribbling with two balls will help you become better at dribbling twice as fast as dribbling with one ball would. Youtube.com is a great place to find two ball dribbling exercises. Here some examples of exercises from Jeremy Lin: