I just got into multi-boxing recently (specifically 2-boxing) after clicking a link to these forums from another forums where I was reading about leveling and twinking.
My characters are Avrana and Avranu both female undead warlocks.
One is specced demonology and uses the voidwalker to tank and the other is specced affliction and uses an imp for some more dps and the stam buff.
Instances so far:
WC - only partial clear at lvl 22
http://files.filefront.com/Dual+Boxi.../fileinfo.html
That's the fraps I have of it. While the actual vid is quickly put together, it still shows how I did it.