  We study the properties of a binary microcavity polariton superfluid
coherently injected by two lasers. The crossover from the supersonic to
subsonic regime, where motion is frictionless, is described by evaluating the
Bogoliubov spectra. We show that according to the Landau criteria, the coupling
between the two components precludes the existence of superfluidity just for
one component but not for the other. By analysing the drag force exerted on a
defect, we give a recipe to experimentally address the crossover from the
supersonic to the subsonic regime.
