  Phonons offer the possibility to connect the microwave and optical domains
while being efficiently transduced with electronic and optical signals. Here,
we present a multimodal optomechanical platform, consisting of a
mechanical-optical-mechanical resonator configuration. The mechanical modes,
with frequencies at 265 MHz and 6.8 GHz, can be simultaneously excited into a
phonon lasing regime as supported by a stability analysis of the system. Both
the MHz and the GHz modes enter a self-sustained oscillation regime, leading to
the intermodulation of two frequency combs in the optical field. We
characterize this platform experimentally, demonstrating previously unexplored
dynamical regimes. These results suggest the possibility to control multiple
mechanical degrees of freedom via a single optical mode, with implications in
GHz phononic devices, signal processing, and optical comb sensing applications.
