A back-of-the envelope calculation showing how close-coupled overlapped airscrews still sweep a greater area than alternative installations. At the same time they allow for a narrower track (and watercraft) altogether and more efficient combinations of motor and propeller.
Though at most efficient when both spin in the same direction, it introduces a bias in steering that could be avoided altogether should they counter-rotate.
Plus the fact drone propellers come in clockwise and anti-clockwise sets, I suppose.
