silver_moon_beam’s design faces a span constraint. Everybody here knows that a monoplane with a span constraint will have minimum induced drag when the span is pushed right up to the limit. If induced drag is your concern, you can "work around" the span constraint by adding another wing (biplane). The most efficient way to do this is to separate the two wings vertically as much as practical (i.e. without adding too much weight or profile drag). The reduction in induced drag for a biplane does not come from interference between the tip vortices as BFoote suggests. In fact, the less interaction between the wings the better.

Here's a simple "thought experiment" that might explain... suppose you are given a fixed amount of area for your wing (or wings) and you want to design a plane with minimum induced drag. Design A has a span equal to 8 and a chord equal to 1. Design B has a span equal to 4 and a chord equal to 2. Everything else being equal, design A should have lower induce drag. Why?

Induced drag comes from putting kinetic energy into the air. If you want to minimize the kinetic energy you are putting into the air, it is better to "push on two air molecules once than the same air molecule twice". If you think of design B as two "closely coupled" wings, this design is essentially pushing on the same air molecules twice to achieve the same effect that design A achieves by pushing on different molecules once (crudely speaking). Geese fly in a V rather than in a column for the same reason (the efficiency they gain is slightly higher because the outboard geese actually take back some of the kinetic energy that the inboard geese put into the air).

Separating biplane wings vertically (increasing the gap) minimizes the extent to which the wings are influencing the same air molecules. Design C with two wings of span 4, chord 1 and infinite gap is more efficient than a biplane with less gap, but it is still not as efficient as design A (geese don’t fly in a “stack” either).

How about adding vertical struts between the tips? Suppose you are given not only a span constraint, but also a height constraint for your design (i.e. your design has to fit inside a box of specified size). It can be shown that the design with minimum induced drag is a biplane with maximum allowed span and height (gap), with vertical struts between the wingtips (incidence of the strut panels is important). Why is this "box" design not more popular? Because the induced drag benefit of the vertical struts is usually offset by the profile drag and weight penalty.