We further examine the semiclassical approach for treating tunneling within classical trajectory simulations by applying it to the dissociation of HN2 and DN2. The calculated level widths of vibrational states are in good accord with the corresponding quantum-mechanical results for a wide range of energies from the tunneling to classical regimes, demonstrating the accuracy of the method. Comparisons of results obtained with normal-mode and efficient microcanonical samplings show that proper sampling of initial states is crucial for the low-lying states in the deep tunneling region, but becomes less important for the states at higher energies. The present work provides further evidence of the applicability of this semiclassical approach to tunneling problems in polyatomic systems.