With canards, tandems, DeLannes, etc. that have coplanar wings, does the foreward wing not complicate the stream for the rearward, at least more than just trivially? If the downward stream of the fore were to feed the underside of the aft (more so than the topside, anyway), would it not create a more turbulent stream for that wing? In the case of the Quickie, I see that Rutan juxtaposed the two wings with +/- dihedral. Perhaps to minimize the above? Or maybe just to offset the lower anhedral (required for landing gear).