Birds produce complex vocalizations through the coordinated dynamics of neuromuscular control, where the syrinx acts as the sound source, and suprasyringeal vocal tract structures function as acoustic filters. In this study, we developed computational models of the avian syrinx and upper vocal tract, reconstructed from high-resolution micro-CT imaging. Using finite element analysis in COMSOL Multiphysics, we introduce dual sources at the bottom of the trachea to analyze the filtering mechanisms underlying biphonic vocalization. The simulation results are experimentally validated using real bird vocalizations. To further elucidate the biomechanical factors governing resonance modulation, we systematically vary tracheal length, glottal radius, and beak angle. Additionally, we perform an analysis of syllable bandwidth to assess the influence of vocal tract dynamics on the frequency range observed in biphonic vocalizations.