Re: double layer??
I do a double layer all the time because I use regular foil and not heavy duty. It makes it stronger to deal with the coals you're going to be putting on top so it doesn't sag into the shisha and burn it because of direct contact.
It won't make less smoke, use the same amount of holes as you have in the past (i use a lot as it is because i like a lot of even air flow) and you'll be fine.