what's wrong with just using a double set of foam mats? I sleep on a single in summer, so that'll be extra comfy and double warm, and from what I read the foam pads have the better r-value anyway, don't get me into trouble with water freezing inside, possibly puncturing, and I really hate to pack these things in the morning as it is, even when it's warm and easy to do.
The only drawback I see with double foam is the size of these things.
