i have always admired aircraft with contrarotating propellers, apparently they are more efficient, but screw that they sound way cooler. many modern aircraft use contra rotating prop, often heavy cargo planes but many nimble fighters such as the westerland wyvern, even variants of the legendary spitfire came equipped with them. personally my favourite is when these gearboxes are built into helicopters such as the Russian series of helicopter designed by kamov. these gearboxes allow helicopters to fly without a tail rotor as they do not experience "torque roll". if you do want to build this, i would recommend using 3d printed nylon or maybe a abs and cnc'ing the gears, alternatively you could try to cast them!