I agree with nwt. Before the 2001 rule change, the regulation was unambiguous -- skydivers were required to use single-harness, dual-parachute systems. Because of poor drafting, the 2001 change addressed only what a single-harness, dual-parachute system consisted of, to distinguish it from a tandem system, instead of requiring its use.
But it doesn't matter. It's been litigated, and the commonly-held (pre-2001) position prevailed against us barracks-lawyer types (or sea-lawyers, if you prefer). The incident was Dwain Weston, Royal Gorge Bridge, 5 October 2003.