I did this to myself earlier in the year.

I tried to just carry a foam pad and the uninsulated pad. I was cold in my 15 degree down bag.

I was successful with a foam pad and a spaceblanket on the ground, wrapping the space blanket around the air mattress (foil side in, if it matters) with a bivy sack on top of that, and a poncho liner wrapped around both me and my 40 degree bag. I was wearing a lot of clothes. I don't recommend this solution from a weight perspective.