Hello Ned. There is no such operating limitation that requires dual action devices. That is in the certification requirements for part 27, Since harnesses are required to be certified, its a moot point. What may be an issue is where you hook it up to. A seat belt evenly divides weight between 2 hard points with minimal slack and therefore shockload. Those hard points aren't rated for a fall, or as a single point. If you are hooked to a certified Class A external load attachment point you are an external load, whether you are sitting on the floor, or on the skid, and you are legal, if you hook it up to a seatbelt hardpoint possibly not, though I've never seen it cited.