The phenomena occurring in gyromagnetic guides in the presence of two propagating modes are analyzed using the coupled mode method. It is found that the superposition of these modes results in the periodic transfer of electromagnetic field energy across the guide. Depending on the structure symmetry with respect to the biasing field orientation, the oscillation period may or may not depend on the propagation direction, but the effect itself has always the nonreciprocal character. It is also found that unlike in the isotropic structures, the difference in phase velocities of guided modes is not a sufficient condition for the energy transfer to occur. The results of the analysis are applied to derive the scattering matrices of a section of asymmetrical structure of two parallel guides coupled via a ferrite medium.