> If not why?
Radiant heat barriers (space blankets) need an air gap between the heat source and the reflector to be effective. A space blanket sandwiched between two polycryo sheets will still conduct heat very well when you lay on it. For non-emergency use, the best plan is to bring sufficient insulation against conductive and convective heat loss to the ground so that there is no significant radiant heat to be lost. (Cold fabric, i.e. the outside of your sleeping bag or the bottom of your sleeping pad, is not a significant source of radiant heat.)
IMHO, space blankets are generally advertised for emergency use because that's precisely when you aren't likely to have sufficient insulation to prevent convective heat loss (i.e., to the wind), so when you wrap a space blanket around yourself you will reduce convective heat loss and have the crucial air gap to allow the space blanket to reduce radiant heat loss from your warm body. That's why you usually see sketches of people hugging their knees: laying down would increase conductive heat loss and remove the necessary air gap underneath the body.
The best place for a space blanket is on the inside bottom of an air mattress (I don't know why manufacturers don't have that as an option; I suppose adding down is lighter?), or slung under a hammock. Just laying on one isn't going to do much for you, IME.
However, two sheets of laminated polycryo might last longer than two individual sheets used one after the other. I have had great success using a single sheet of polycryo, but on the last night of a recent trip a two-inch tear in the edge of the sheet instantly turned my large polycro ground cloth into two odd-sized small polycryo ground cloths when I shook it out :(