Would supersonic bi/tri/multiplane wings have less lift interference drag than a subsonic version?
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