  We investigate spin squeezing of a two-mode boson system with a Josephson
coupling. An exact relation between the squeezing and the single-particle
coherence at the maximal-squeezing time is discovered, which provides a more
direct way to measure the squeezing by readout the coherence in atomic
interference experiments. We prove explicitly that the strongest squeezing is
along the $J_z$ axis, indicating the appearance of atom number-squeezed state.
Power laws of the strongest squeezing and the optimal coupling with particle
number $N$ are obtained based upon a wide range of numerical simulations.
