For a Bose-condensed gas in a combined potential consisting of an axially-symmetric harmonic magnetic trap and one-dimensional (1D) optical lattices, the motional interference side peaks along the axis direction are formed after the combined potential is switched off. Based on this physical system, we make a new experimental suggestion which can be realized with current experimental technique to investigate the side peak superposition and discuss the second interference and amplification effect of a Bose condensate. In the region where the side peaks undergo constructive interference, the density of condensed atomic cloud will be amplified effectively.