I have read about people using old double hose regs, but that was not my question. Why doesn't anyone make new ones?
Because there is no percieved market for them.
The companies, USD etc would have had to do some engineering work to add a LP and HP ports and they apparently did not know how, although Luis does, lol.
Scuba training changed radically in the 70s over the course of the decade from stringent NAUI/YMCA courses that lasted weeks to PadI one day wonders. In 1970 nobody used or knew to use an octopus and there were no BCs in use and by the end of the decade BCs were required, octos required, spgs required. In the 60s we did not largely use spgs, we used J valves.
Training standards changed to rely upon these new items and sharing air with an OOA diver via an octopus relegated Buddy Breathing to the dust bin of history.
Purge valves were integral to the way PadI teaches a regulator hand off. Double hose regs have no purge valve.
Double hose regs do breath very well but they are more positionally sensitive and require better technique which cannot be taught in today's one night scuba "lesson". (As does diving without a BC)
In the 70s the premier companies, Voit, USD, Nemrod, Sportsways, Healthways essentially disappeared, fell into disfavor or morphed into today's companies, et al.
USD > AL
Healthways > Scuba Pro
White Stag became Speedo and you guys will not wear their suits, lol
Voit, lol, they seem to make soccer balls in Mexico, dunno.
Scuba diving changed from a man's adventure sport to a family hobby. Local diving was de-emphasized and the scuba vacation industry was born with glitzy adds and exotic destinations ruined by hordes of Wall Mart shoppers in Scuba Steve outfits.
All of these things conspired to bring twin hose production to an end by about 1973 or so, debated, but Nemrod continued until I think 1998 and the US Navy has had AL make an amazing, modern, stunningly excellent training regulator for their use only recently called the Mentor. AL denies it. Only one of these is in civilian hands so they do exist.
AL did release a POC junker, poorly engineered, ugly as sin, double hose a few years ago called the Mistral. Most of us were not impressed with it and it died.
Does this answer your question?
N