The injection of two primary waves, with possibly different frequencies, into a plasma can result in strong nonlinear interactions if the additive or subtractive frequencies of the two waves is equal to the two-ion-hybrid or lower hybrid frequencies. The in situ generation of waves at the linear-resonant frequencies effectively constitutes an “internal antenna,“ which may alleviate the undesirable surface interactions and reflections resulting from the direct injection of waves at the linear-resonant frequencies into a plasma. It is shown that simultaneous fundamental, second-harmonic, and third-harmonic heating can be achieved in a fusion device by an appropriate choice of frequencies for the two primary waves.