I've dual boxed 2 locks to 68, and they are a lot of fun.

Up until Outland I had them both specced Destro, but after a little research I switched them to both to Demo and OMFG what a huge speed increase it gave me when questing.

The main runs a Voidwalker, while the slave runs a Felguard. Mobs are always kept at distance by the 2 pets while my girls dot-dot-sb in complete safety. If shit hits the fan I can use the Voidwalker's AOE taunt.

And as a bonus Fel Armour returns the health sacrificed when you Life Tap, there is no downtime at all.