If you're not being boosted, questing is the fastest leveling especially if you quest-stack.
Two of the same class will be a lot easier to box, but two classes is not especially difficult.

If you were to cover the tank and healer role, you'd have the basis for a group as dps are easy to find, while generally tanks are harder. If you went this route, I'd suggest a healer who can dps well and a tanking class that can dps spec, at least for the leveling up stage.