A semiclassical study is made for the complete transmission and the complete reflection phenomena in two coupled molecular potential systems. The conditions for these phenomena to occur are expressed analytically in terms of nonadiabatic transition probabilities and phase integrals, which can be provided by the semiclassical theory. We also introduce an exactly solvable analytical model of diabatically avoided crossing, in which two diabatic potentials coupled by a constant coupling are close together in a certain spatial region. These models and phenomena may be useful in controlling various molecular processes in laser fields, since in the dressed or Floquet state formalism molecular potentials can be shifted up and down and are made to cross or avoid crossing with other potentials.