The dependence of the Mossbauer line shape on the double-exchange interaction is studied for dimeric mixed-valence systems in the framework of a model that treats the entire nuclear-electron system quantum-mechanically. The coherent tunneling motion of the "excess" electron between the metal sites produces a resonance splitting of the localized states. In the case of weak double-exchange interaction, this motion shows up as a splitting of Mossbauer lines. This effect however, is readily suppressed by an asymmetry distortion, which destroys the resonance conditions. The origin of different patterns in the temperature dependence of the Mossbauer line shape is discussed.