Induced drag reduction from tandem wing
The downwash is about as high as wide, and angled only by a few degrees (more at slow speed). So the longitudinal distance needs to be many times longer than the wing span to make the wings independent.
And it would still be less efficient than increasing the span:

If you double the span, you will decrease induced drag four times.
If you half the lift, you also decrease the induced drag four times, but then you add the second wing to get your lift back and that also doubles the induced drag, so you have half the induced drag, not quarter.