For starters, I'd definitely do the DRK/WHM character. DRK is a heck of a lot of fun, and being able to switch roles there will probably be extremely helpful, as others have said. You'll eventually want to level GLD and MRD anyway for the shared cooldowns, but they aren't 100% essential for leveling content (except Provoke. Get Provoke ASAP if you haven't). That other character will be really good to level all the tanks with, too, since it will have all of the defensive cooldowns it could need (and become the fabled triple-tank).