An interesting paper states that biplanes have less induced drag than monoplanes:
"The induced drag of a multiplane is lower than that of a monoplane of equal span and total lift because the nonplanar system can influence a larger mass of air, imparting to this air mass a lower average velocity change, and therefore less energy and drag. For a biplane, if the two wings are separated vertically by a very large distance, each wing carries half of the total lift, so the induced drag of each wing is 1/4 that of the single wing. The inviscid drag of the system is then half that of the monoplane." see paper here:http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf
The paper also suggests building a biplane with cantilever wings instead of the usual strut and wire truss. This has been done before with Quickie and others.
But the paper also states that a biplane would have half the reynolds number of a monoplane of same span.
Could a cantilever biplane ultralight be almost as efficient as a monoplane with the same span and twice the wing chord (same area for both)?