  The spontaneous breaking of time translation symmetry in periodically driven
Floquet systems can lead to a discrete time crystal. Here we study the
occurrence of such dynamical phase in a driven-dissipative optomechanical
system with two membranes in the middle. We find that, under certian
conditions, the system can be mapped to an open Dicke model and realizes a
superradianttype phase transition. Furthermore, applying a suitable
periodically modulated drive, the system dynamics exhibits a robust subharmonic
oscillation persistent in the thermodynamic limit.
