I don't think they can balance around both. Considering how he will get Full XP, even in the duo lane. He automatically gives his team an over all XP advantage Just by Duo laning.
Though I don't know how you could make him duo otherwise. To duo, you basically can't be level dependent, and rely more on your items. Which enevitably means you rely more on Auto attacks rather than spells. This is why the ADC is Iconic in the Duo lane, cause they can afford to XP share with a support. And this Mord Duo lane only works because he cheats, getting the same XP as if he was in a solo lane.