Two edge-emitting lasers mutually coupled through orthogonal optical injection exhibit square-wave oscillations in their polarization modes. The TE and TM modes within each individual laser are always in antiphase, but the TE mode of one laser leads the TM of the other by the one-way time of flight between lasers. The duty cycle of the square waves is tunable with pump current and coupling strength, while the total period remains close to the roundtrip time. Numerical simulations give similar results and reveal the role of noise in stabilizing the oscillations.