Chirality, a fundamental property of nature, exists extensively across various scales, ranging from microscopic molecular structures to macroscopic biological morphologies [[1], [2], [3]]. The intrinsic chirality of substances in nature is often extremely weak, resulting in very faint chiral optical response signals, which pose great challenges to detection efforts. With the continuous advancement of research on optical metasurfaces, chiral metasurfaces have drawn extensive attention due to their unique properties to support intrinsic chirality under normal incidence [[4], [5], [6]], while exhibit extrinsic chirality under oblique incidence [[7], [8], [9]]. Combining the advantages of high integration of optical metasurfaces and the ability to enhance light-matter interactions, chiral metasurfaces have demonstrated tremendous application potential in numerous fields, such as chiral light emission [[10], [11], [12]], biological detection [[13], [14], [15]], and polarized light modulation [[16], [17], [18]]. Chiral metasurfaces can generate strong optical chiral responses. However, the Q-factors remain low due to significant radiative and non-radiative losses, which severely limits the intensity and efficiency of intrinsic chiral interactions between light and matter.

To overcome these limitations, the physics of BICs has recently been extended to photonic systems, enabling ultra-high Q resonances that significantly enhance intrinsic chiral light-matter interactions. As eigenstates embedded in the radiative continuum, BICs stay perfectly localized, offering theoretically unlimited Q-factors and dramatically amplified light-matter coupling [[19], [20], [21]]. Moreover, chiral Q-BICs display singular spin-selectivity: they are transparent to one circular polarization yet resonantly amplify the opposite spin state. Theoretically, they can achieve maximum circular dichroism spectra and extremely high Q-factors [[22], [23], [24], [25]]. This spin-locking feature positions chiral Q-BICs as a versatile platform for high-efficiency nonlinear spin-optical devices [[26], [27], [28]] and ultrasensitive chiral biosensing [29,30].

Recently, the chiral metasurface based on BICs has attracted attention from researchers and been further advanced. To simultaneously realize significant high-order nonlinear CD and strong high-order nonlinear responses, Liu et al. theoretically proposed efficient high-order harmonic generation and corresponding high-order nonlinear chiroptic effects in a Si-based metasurface based on the accidental BICs [31]. The Q-BIC metasurfaces with dual-band chiral properties reported by Cao et al. are mainly based on silicon ellipse pairs and other single-layer geometric units, which have limited control over the frequency position of chiral resonances [32]. Cai et al. demonstrated the split and displacement of topological charges through tilted double-elliptical half-disk metasurfaces [33]. Zhang et al. proposed an all-dielectric chiral metasurface supporting two chiral Q-BICs [34]. However, whether it is a single band or multi-band chiral metasurface, their spectral positions are fixed and the structural scalability of the devices presented is limited. The design of bilayer all-dielectric systems for designated dual-band high-Q chirality remains underexplored. Accordingly, we introduce a bilayer metasurface platform where two chiral bands can be independently engineered and combined, offering scalable structure and flexibility in multi-band chiral light manipulation. The calculated results show that the proposed metasurface not only markedly enhances the modulation depth of chiral optical responses but also enables flexible optimization of multi-band CD magnitudes and Q-factors by simply tuning the interlayer parameters. This study establishes a general design framework for multi-band chiral nanophotonics and paves the way to deploy chiral metasurfaces in practical application and multi-functional scenarios.