A general symmetry-controlled mechanism is proposed for realizing dual flat bands of bound state in the continuum (BIC) and its radiative counterpart in photonic crystal slabs. By breaking the vertical mirror symmetry of a slab, interpolarization coupling between TE-like and TM-like modes is activated, while intrapolarization coupling among modes within the same polarization class is simultaneously preserved. The cooperative action of these two coupling channels gives rise to the concurrent flattening of both the BIC-hosting band and the radiative band, resulting in a dual flat-band system with strongly contrasting quality (Q) factors. An effective two-step coupling model is constructed to capture the essential physics and show that the emergence of the flat bands is governed by geometric tuning rather than accidental degeneracies. The mechanism is shown to be generic with respect to polarization and material platforms, enabling dual flat-band states in both low- and high-index systems, with substantially enhanced angular bandwidths in the latter. These findings establish a unified route for flat-band photonic engineering and provide a robust platform for angle-tolerant resonant photonic functionalities.

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• Flat bands
• Photonic crystals
• Mode coupling theory