While the duals may mitigate the movement of hot/cold air into and out of the unit one has to take into consideration of the thin, poorly insulated walls, floors and ceiling.
IF the insulated portions were heavily insulated then the duals would be worth the extra expense. All in all there are more square feet of poorly insulated areas than window areas, regardless of duals, triples or singles. This, therefore, mostly negates the need for heavier windows. Just think of the skylights, the refrigerator, water heater, furnance, fans and the other openings to the outside that leak air.
This is just my take on the subject however.