For tasks requiring robot-environment interaction, stiffness control is important to ensure both stable contact motion and collision safety. The variable stiffness approach has been used to address this problem. We propose a hybrid dual actuator unit (HDAU) which is a novel variable stiffness unit design. The HDAU is composed of a hybrid control module based on an adjustable moment arm mechanism and a drive module with two motors. By controlling the relative motion of gears in the hybrid control module, position and stiffness can be simultaneously controlled for the same joint. The HDAU provides a wide range of joint stiffness due to nonlinearity obtained from the adjustable moment arm. The joint stiffness can be kept constant independent of the passive deflection angle of the output shaft. Furthermore, stable interaction can also be achieved because the joint stiffness is indirectly adjusted by position control of the hybrid control module. The characteristics of the HDAU are analyzed in this study. We show by experiment that the HDAU can provide a wide range of stiffness variation and rapid response for stiffness change.