Induced drag is proportional to the square of span loading. The normal method to decrease it would be to increase span. This, however, leads to an exponential weight increase.
So, my question is, if tandem wings are placed with a large longitudinal gap, so as to avoid downwash, would they act as independent wing spans sharing the weight of the aircraft to reduce induced drag? If so, how large would this gap need to be? Would one at the nose and another at the tail be enough?