ActionBarTargeting

ActionBarTargeting is a WoW addon that automatically creates targeting and follow macros for a multibox team of characters. The targeting system is based on the actionbar conditional (active actionbar page), so focus is kept free for defining crowd control targets. If you want it for a particular encounter (e.g., vehicle quests), focus targeting is also supported and can be switched on/off even in combat.

Full Documentation: Google code

Discussion Thread: dual-boxing.com

Note that you also have to install the companion addon MultiboxRoster. See the setup instructions at Google Code or in the readme.