Cross-sensitivity inevitably degrades the detection performance of multi-channel mass sensors and thus restricts their practical deployment in industrial environments. To suppress this parasitic effect, we herein propose a symmetric “mountainshaped” resonant beam architecture founded on the principle of mode localization. Firstly, the resonant beam structure is theoretically analyzed and the dynamic equation is established. Then, the specific dimensions and the first three mode shapes are determined by COMSOL finite element software simulation. Secondly, the influence of adding adsorption mass on different resonant beams on the first three frequencies and amplitudes is verified by experiments. Then, the amplitude ratio is used as the output signal to achieve single-mass detection of the first three modes. On this basis, a dual-mass detection scheme is further designed and experimentally verified. The results show that the mass detection range of 0~16, 0~4, and 0~3 mg can be achieved at the first three frequencies, respectively. By exploiting the disparate modal shapes of the second and third eigenmodes, a decoupled resonant sensing paradigm for dual-channel mass determination is established. The synergistic exploitation of the second- and third-order eigenfrequency pairs effectively nullifies the deleterious influence of cross-sensitivity on dual-channel measurements, thereby enhancing the robustness and reliability of the proposed methodology. These findings furnish a rigorous theoretical basis for the subsequent design and optimization of high-performance mass sensors.