What is the most economical material to cover a hoop greenhouse? Would 2 layers of 4 mill plastic sheeting ehlop more than one?

An air space between the two, yes, otherwise all it would do is help a little if something made a hole in the other one.
Air is actually a pretty good insulator. If you could figure out some way to make an inch in between, that would save some of the sun's heat - for a while. Not at night, if you are in the north. I'm not sure it's worth it, because you want the sun to get through, and the plastic is a bit cloudy and will cloud up more.