  We study the phase-sensitivity of collisional phase-diffusion between weakly
coupled Bose-Einstein condensates, using a semiclassical picture of the
two-mode Bose-Hubbard model. When weak-coupling is allowed, zero relative phase
locking is attained in the Josephson-Fock transition regime, whereas a $\pi$
relative phase is only locked in Rabi-Josephson point. Our analytic
semiclassical estimates agree well with the numerical results.
