Re: Double foil
Single or double foil is pretty much a personal preference, and also depends on what you're smoking, the coals you're using, etc. Try both and see what works best for you. It's just two different schools of heat management.
However, I agree with sabsz that this likely won't fix your problem with low taste.
If you're using a juicy shisha, have you tried stirring it up a bit before you pack it to get more of the juices?