Disclosed is a gearbox with two driver gears that are phase shifted 180 degrees with respect to a driven gear so that counter-rotation of the driver gears combine to rotate the driven gear together. The gearbox can be used to couple a counter-rotating, coaxial wind turbine rotors by coupling the driver gears to the counter-rotating rotors and the driven gear to a generator so that the counter-rotating rotors on the wind turbine rotate the driven gear in the same direction. The driven gear can be coupled to a generator to produce electricity.