Service robots are often required to be able to perform both the position control and force control for various tasks. In addition, service robots should cope with collision safety to prevent possible damage to humans and environments during unintended collision. To this end, several redundant actuation systems have been proposed. In this research, the Dual Actuator Unit (DAU) was developed to improve the manipulation efficiency by maximizing the velocity or payload depending on various situations. The DAU is composed of two actuators connected in series through a planetary gear train and a cam-based clutch system. In this paper, the characteristics of the DAU having three operating modes - dual actuation mode, high speed mode and high torque mode - are discussed. Various experiments show that the DAU can carry out the given task flexibly and it can improve the manipulation efficiency.