Two control systems are provided to run in parallel between a surface location and the final controlled element, in this case a subsurface safety valve. The primary control circuit is controlling until a predetermined signal is given to the secondary control line which has the effect of actuating the valve a single time against the force of a bias or a shear pin that breaks. The movement of a shuttle in the housing due to the predetermined signal being provided in the secondary line puts the secondary control system in the position of running the tool. The primary system is valved off and cannot return into service. There are just two lines into and out of the housing to make the valve operate at the desired location. A test port is provided for surface testing.