Two layers of plastic can be utilized with some modifications. The primary problem, however, is that there is no seal on the bottom edge of the two plastics which allows air to escape out of the bottom edge. Secondly, the ropes used to hold the plastic taut and in place press the two layers together making it impossible for air to pass. But this can be overcome.

Improved insulation with two layers of poly and air from a blower can be accomplished by purchasing additional spring wire and adding a hipboard or baseboard. This removes the necessity for the rope as the spring wire will hold the plastic down. However, the rope can be used to secure the roll-up side curtain, if you choose to install a hipboard.

Keep in mind the two layers of plastic only benefit when air can pass between them. So, when using two layers of plastic without the blower the benefit of additional insulation is lost and the further transmission of light is diminished.