Aerial cooperative system with suspended cables plays an important role in material transportation, disaster relief and other related fields. To achieve safe and rapid conveyance, a motion planning method for dual-UAV system is proposed in this paper. Utilizing the representation of piecewise polynomials, a nonlinear optimization problem considering various constraints is formulated. Subsequently, a feasible payload trajectory is obtained, based on which the cable force allocation is conducted by solving two quadratic programming problems. Finally, with the help of differential flatness, the entire system state is attained. Simulation results demonstrate the effectiveness of the proposed scheme.