  We investigate the transport and the dynamical properties of tunnel-coupled
double charge shuttles. The oscillation frequencies of two shuttles are
mode-locked to integer multiples of the applied voltage frequency $\omega$. We
show that left/right-symmetric double shuttles may generate direct net current
due to bistable motions caused by parametric instability.
  The symmetry-broken direct current appears near $\omega =\Omega_{0}/(2j-1)$,
($j=1,2,...$), where $\Omega_{0}$ is the dressed resonance frequency of the
relative motion of the two shuttles.
