These are nice. As a young boy I used to play around with rolls of Ektachrome ina camera my Dad gave me. To double expose, the ASA rating would have to be really low right? sub-100 perhaps?
Old tech - I had a hobby of shortwave listening. I am still chasing an analogue sony shortwave radio but they are super expensive. I now have a tiny Techsun radio and aim to get a better one sooner or later. There are still broadcasters out there. It's a dying thing beiong replaced by the internet of course. BUt there are also digital shortwave technologies in the pipeline. DRM Mondial for instance, so it is a form of media that has use cases and will probably evolve. DRM raedy radio units are pricey at the moment though.
It depends on where you're shooting...If you wanted to double expose in full daylight outdoors, you would definitely need low speed film, and probably a pretty high f-stop and shutter speed to compensate. I was shooting indoors with ambient light, and in that case the 100 film was fine. I don't think an instant film under 100 was even available on the market at the time.
shortwave radio, that's a super interesting hobby! maybe as the digital technology becomes more popular the old sony radios will go down in price? As digital took over I ended up getting some really great film cameras for really cheap. unfortunately I cant find anywhere local to get film processed anymore...and my house is too small for a darkroom. One day perhaps!