Whether you inflate 2 layers or not is up to you. *The greenhouse polyfilm will work without inflating. *But you will have less insulation rating. *You would know better how long your cold season is and how warm you want to heat it as well as your utility costs. *
If you decide you want to look into 2 layer inflated, click on the IGC poly film link and you will also see other things such as inflation blowers and aluminum rails with clips that lock the 2 edges of the poly together. *These rails go around the entire perimeter on the outside of the structure so the 2 layers can then be inflated like a pillow. *You can do it less fancy and simply overhang the plastic and then roll up the excess around a piece of wood like a windowshade.. and then secure that down. As long as the edges are sealed to keep air from escaping.
Of course your other options would be a clear fiberglass panel or more fancy.. twin wall polycarbonate. *Fiberglass is not too expensive. *It would be a bit more than the polyvinyl sheet but might look a little nicer and won't flap around in the wind (a potential problem if you use uninflated polyvinyl sheet that is not pulled snug enough). *Twinwall polycarbonate looks great and has higher insulating value than inflated polyvinyl but is ALOT more expensive. *Fiberglass and polycarbonate will also last about twice as long compared to polyvinyl sheet.