When reflective layers touch they become effectively worthless. A reflective layer is usually thin and metallic. That means they have high conductivity and very little heat capacity. They will quickly equilibriate with what ever they are touching. If you lay a Neoair on a reflective layer, the two will quickly become the same temperature. It won't matter what the reflective property of the layer it is sitting is since the radiation exchange is occurring inside the mat, thus only the reflective properties of the mat matter. Because of conduction the space blanket's radiative properties are moot. Now you proposed, a bubble mat with a reflective layer. The bubbles are designed to reduce conduction between the top and bottom. However you do indeed get a diminishing return on each additional thermal layer. I suspect Neo Air's already use multiple radiation layers inside the pad(at least the new ones), so you probably won't get much additional benefit regardless.

I can go into the equations if you are interested.