  We address light propagation in couplers with longitudinal biharmonic
modulation of refractive index in neighboring channels. While simplest
single-frequency out-of-phase modulation allows suppression of coupling for
strictly defined set of resonant frequencies, the addition of modulation on
multiple frequency dramatically modifies the structure of resonances. Thus,
additional modulation on double frequency may suppress primary resonance, while
modulation on triple frequency causes fusion of primary and secondary
resonances.
