This paper focuses on the manipulation of multiple objects, for example, to rake up many objects on a table, lift them up and put them in order, which is difficult to perform for a single manipulator system. This kind of operation becomes possible or much more effective by the cooperation of two manipulators. The lifting up operation of multiple objects by putting them between the two end-effectors is discussed. The use of visual or touch-sensory feedback for the operation, as humans do, cannot prevent the loss of balance for the manipulator system. For the purpose of safe operation, the kinematic analysis and a method of predetermining the pushing force are proposed. The experimental results indicate the effectiveness of the proposed method using simulations with various estimated errors.