I might think about what duo would be most efficient at training skills and moving around. I don't know about other folks but with the 3x XP quests I find that a proportionally large amount of my time is spent going and getting trained.

Also class-specific quests are a pretty major slowdown on the leveling process in my experience.

Because of these two factors I would think that leveling two mages should be looked at as they can easily port to a trainer and hearth back and they don't have class-specific quests that need to be done for stances or totems or forms or whatever.