Extensive wind tunnel testing done in the 1920s and early 1930 showed that using virtually the same incidence on both wings of a bipe would maximize lift/drag. Theory indicated that slightly negative incidence for the upper wing, relative to that of the lower wing should be most efficient, but tests did not support this. Somewhere around one degree more positive incidence for the upper wing seemed best, but the improvement was hard to measure. I often use about one degree less incidence in the upper wings of my bipes - seems to help aerobatic line holding. I went into this in a bipe tech series I did for M.A.N a few years ago.