We investigate how interlayer coupling shapes high-harmonic generation (HHG) in AA-stacked bilayer ${\mathrm{WS}}_2$ using real-time time-dependent density functional theory and semiconductor Bloch equations. The bilayer harmonic yield is found to depart from the coherent layer-number enhancement expected for independent layers, with the departure growing continuously as the interlayer distance is reduced. We trace this behavior to coupling-induced band splitting, which opens two electron-hole recombination pathways whose relative dynamical phase is set by the interlayer coupling. As the coupling increases, the two pathways evolve from nearly constructive toward partially destructive interference. Time-frequency analysis and two-color field probing show that their recombination timing shifts accordingly. Interlayer coupling thus provides continuous structural control over recombination-pathway interference, allowing the amplitude, phase, and timing of solid-state high-harmonic emission to be tailored.

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• Nonlinear optics
• Strong-field-induced spectra
• XUV generation