  We formulate a generic concept of engineering optical modes and mechanical
resonances in a pair of optically-coupled light-guiding membranes for achieving
cascaded light scattering to multiple Stokes and anti-Stokes orders. By
utilizing the light pressure exerted on the webs and their induced flexural
vibrations, featuring flat phonon dispersion curve with a non-zero cut-off
frequency, we show how to realize exact phase-matching between multiple
successive optical side-bands. We predict continuous-wave generation of
frequency combs for fundamental and high-order optical modes mediated via
backward- and forward-propagating phonons, accompanied by periodic reversal of
the energy flow between mechanical and optical modes without using any kind of
cavity. These results reveal new possibilities for tailoring light-sound
interactions through simultaneous Raman-like intramodal and Brillouin-like
intermodal scattering processes.
