Phononic frequency combs are equidistant spectral ladders generated when a driven vibrational mode transfers energy to an autoparametrically excited subharmonic mode through anharmonic intermode coupling. In previous studies, the coupling strength governing this energy transfer has typically been treated as a free parameter, extracted from a fabricated device, or adopted from an earlier calculation for a specific mode pair. As a result, whether a given crystal can support a phononic frequency comb, and the drive strength required to generate it, has not been predictable a priori. Here we obtain the complete parameter set from density functional theory. Twisted bilayers of and at the commensurate angle 21.787^∘ adopt space group P3 in a 42-atom moiré supercell. The second-order force constants provide the mode frequencies; the third-order constants provide the linewidths through the Boltzmann transport equation and hence the quality factors; and the same third-order tensor projected onto the phonon eigenvectors of a chosen mode pair provides the anharmonic couplings kt and kss. Both bilayers constitute a mode pair within 0.8% of the 2:1 resonance with ω₁/ω₂ = 1.9923 in and 1.9943 in . By integrating the resulting two-mode equations, both modes generate combs through a discontinuous transition: at 701 fth in the envelope variance rises from 2.1×10³ to 1.37×10⁴ and the tooth count of mode 1 from 7 to 16 within a single 5 fth increment, identifying the onset as subcritical. The spectral entropy peaks before this jump, at 341 fth. The two compounds carry comparable coupling, differing by a factor of 1.2. However, the comb excitation threshold is set by the quality-factors of signal modes: Q₂ = 672 for and Q₂ = 3844 for .