This paper proposes a design for a knee-ankle prosthesis which ultimately controls two linear hydraulic actuators using a single pump. The design takes advantage of the fact that the knee and ankle never produce large positive powers simultaneously during stair-climbing, and that the ankle produces more positive power than the knee during walking. This makes the prosthesis lighter and smaller. During walking, the ankle joint adds forward propulsive power similar to that of an intact ankle. During stair climbing, the knee joint provides assistance like that of an intact knee. The hydraulic nature of the prosthetic allows it to function in a semi-active mode, where it behaves similar to a microcontroller controlled passive transfemoral prosthesis.