Seems that if you add a second wing, and then do all the other things (increasing wingspan, increasing chord to increase area without increasing span, etc) you still have induced drag, which gets greater with wing area, be it wider chord or longer span, and weight, which increases whenever you add anything. Take a lesson from the soaring crowd: how many biplanes do you see in the ranks of sailplanes?

Fact of aerodynamics: biplanes are less efficient flyers than monoplanes (or gliders). History has borne this out.