  We study the dynamic structure factor of two coherent bright solitary waves
in attractive Bose- Einstein condensates confined in a harmonic trap. We
demonstrate that the wave function of the two solitary waves with a fixed
relative phase shows interference in momentum space. The fringes are shifted
depending on the values of the phase. This momentum interference can be
extracted from the dynamic structure factor of the system using stimulated
two-photon Bragg scattering. Thus our method provides a way to measure the
relative phase directly.
