Dual‐band plasmonic nanoantennas, exhibiting two widely separated user‐defined resonances, are essential for studying and optimizing plasmon‐enhanced optical phenomena, including photoluminescence, Raman scattering, and nonlinear effects such as harmonic and sum‐frequency generation. The nanoparticle‐on‐slit (NPoS) or nanoparticle‐in‐groove (NPiG) antenna is a recently introduced dual‐band structure with independently tunable resonances at mid‐infrared and visible wavelengths. It has been used to enhance sum‐ and difference‐frequency generation from optimally located molecules by an estimated 1013‐fold. However, theoretical understanding of its eigenmodes remains limited, constraining further optimization and broader application. Here, the quasi‐normal modes (QNMs) supported by NPoS structures are investigated, analyzing how both near‐field (giant photonic density of states) and far‐field (radiation pattern) characteristics influence upconversion. Tuning strategies are identified to adjust visible and mid‐infrared resonances independently while maintaining strong near‐field mode overlap, which governs the efficiency of nonlinear processes. Additionally, mode analysis reveals a previously unexplored resonance offering greater field enhancement and superior spatial mode overlap with the mid‐infrared field, potentially improving upconversion efficiency fivefold compared with the existing results. This work helps to rationalize and optimize the enhancement of nonlinear effects across a wide spectral range using a flexible and experimentally attractive nanoplasmonic platform.