I think this type decision is somewhat dependent upon the type of diving you are doing. If you're diving less than 33 feet, you probably do not need an alternate air supply. Simply surface. If you are diving between 33 and 66 feet sea water, it would be good to have some kind of alternate air supply to get you to the surface. A pony bottle, or even (heaven forbid, a spare air), would do the trick here without overhead hazards. I have some doubles, and really enjoy their fit and feel. I use one set with a manifold that provides for duel regulators, and sometimes mount a double hose regulator on the center mount (older, Sherwood style), and a single hose as a backup. I also dive twin 50s with a regulator with an octopus, and don't worry about the duel function. The vast majority of my dives are less than 33 feet, however.
The DIR people are very specialized, and dive deep, decompression dives in wrecks or in caves. Here, the twins with two regulators, separate SPGs, etc., are excellent choices. But do not be deceived; DIR divers who dive this way do so because they cannot simply surface! To do so risks life-threatening decompression sickness unless they complete their staged decompression. Usually, they do not dive from a boat or shore facility that has a recompression chamber (a must for decompression dives by some authorities--JY Cousteau and the US Navy, for instance). So they must, absolutely, have a dependable alternative. However, they are on the "cutting edge" of safety, and something as small as a malfunctioning mouthpiece can mean the difference between life and death. This is not "sport diving" in the sense of the word I know, and IMHO should be avoided unless you really, truly have a very, very good reason for doing this kind of diving.
SeaRat
