  We analyze the collective dynamics of an ensemble of globally coupled,
externally forced, identical mechanical oscillators with cubic nonlinearity.
Focus is put on solutions where the ensemble splits into two internally
synchronized clusters, as a consequence of the bistability of individual
oscillators. The multiplicity of these solutions, induced by the many possible
ways of distributing the oscillators between the two clusters, implies that the
ensemble can exhibit multistability. As the strength of coupling grows,
however, the two-cluster solutions are replaced by a state of full
synchronization. By a combination of analytical and numerical techniques, we
study the existence and stability of two-cluster solutions. The role of the
distribution of oscillators between the clusters and the relative prevalence of
the two stable solutions are disclosed.
