If you aren't wanting to spend a few days on each character. i would say Do them at the same time. But if you are looking to Get both of them end game ASAP! Multiboxing would be decent. as the person above me said aswell. you could level one and Power level the other. Really does depend on how you are wanting to spend your time leveling each one. Might i ask the Class of each one? also what Realm and expansion are you playing?
-Regulise
Druid (main toon) as feral. Paladin (as ret but with healing gear, since no real good talents early on)
So I can aoe kill with bear and swipe, and heal with pally with no down time