In this paper, we study the attitude stability and the disturbance attenuation properties for an underactuated spacecraft equipped with two parallel CMGs. We first present the actuator configuration and then compare this with typical fully actuated CMG-based configurations. The system model is then derived. Considering the pointing of an underactuated spacecraft, we derive a Lyapunov control law which is further modified to a dissipative controller to account for disturbances. Simulations are included to demonstrate the effectiveness of the proposed control law to attenuate perturbations and to render the underactuated attitude closed-loop system to a dissipative system.