Biplane wings interfere with each other, reducing the total lift often to about only 20% more than a mono wing of the same chord and span.
But at supersonic speeds - say, Mach 2, where Mach angle is 30 degrees, the wings (now provided with supersonic airfoils, before anyone asks) would have to be 0.5 chord-lengths apart for their shockwaves to interfere.
So, am I correct in saying that for a given chord-length separation, drag due to airfoil interference on vertically stacked multiplane wings interference decreases with Mach number?