Would supersonic bi/tri/multiplane wings have less lift interference drag than a subsonic version? 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?
Amazingly, at supersonic speeds it can work the other way. In the Busemann biplane the shock waves interfere in such a way as to actually reduce drag and also to reduce the sonic boom. Wide enough separation to avoid interference would actually increase supersonic drag.

See for example:


Wu, Jamieson and Wang
Kusunose, Matsushima and Maruyama