We study the dispersion and electric field distributions of magneto-plasmon polaritons propagating in thin films of conductors separated by a thin insulating dielectric film. The system supports guided modes, which exist at frequencies and wavenumbers generally forbidden; the coupled surface plasmon polaritons of the system exhibit propagation which is non-reciprocal with the application of a transverse DC magnetic field. The forbidden guided modes are symmetric with respect to propagation direction, and therefore the combinations of effects offer interesting device application possibilities.