Considering the coherent nonlinear dynamics between two weakly
linked optical waveguide arrays, we find the first example of
coexistence of Josephson oscillations with a novel self-trapping
regime. This macroscopic bistability is explained by proving
analytically the simultaneous existence of symmetric,
antisymmetric and asymmetric stationary solutions of the associated
Gross-Pitaevskii equation. The effect is illustrated and confirmed
by numerical simulations. This property allows to conceive an
optical switch based on the variation of the refractive index
of the linking central waveguide.