Yeah I like the standby lamps too. They're a little hard to find though. I've never seen them used around here. They cost a lot more than normal lamps, probably because of lower demand. Can't be much more in materials since nothing is added but the second arc tube. Less material than two separate bulbs!
Yeah I don't think we used these here even back when everything was HPS. Kind of a interesting concept. I think in Japan they have lamps with twin MV arctubes and and even stranger one with a HPS and MV arctube. I believe that one needed a special 3 contact socket.
I remember reading somewhere that in Japan they have hybrid HPS-MV streetlights in some areas. In foggy weather or winter, the HPS is used but all other times the MV is used.