I think folks are missing the point. The attractiveness of converted industrial engines is not power or efficiency, it's availability and low cost. Once you get into modifying or hopping up the industrial engines to any degree, you've lost a whole lot of people that wouldn't be comfortable doing much more than bolting on a redrive and propeller. This tandem twin idea is fun to explore, but the real market would be a relatively simple, easy-to-build design that could carry a large adult on just 25 hp or so. That would mean a motorglider-like design in the single-seat microlight (300 kg gross weight) category, perhaps along the lines of the Airsport Song but simpler. If you really want a twin then it's not a big jump from that to a baby Schweizer Twin Condor.