As I recall the dual valve springs were a Chevrolet Performance Option. The two springs increase the spring pressure to reduce float at high RPM's on the heavy valve train, but the main function is that the two springs rub against each other and act as a damper. The second spring also acts as a safety measure. Broken springs can be found before they drop a valve into the engine. They're available from Summit, Comp Cams, Chevy Performance, Speedway and a host of others.