Hi Ohoopee,

I use both a single and a double, my area generally doesn't get cold weather lower than 15 - 20 F at night. I live on the southeast coast, U.S.

I don't find the second layer helps with R value or reducing wind chill on it's own, but if i get some insulation between the layers it definitely does.
I have a smallish synthetic quilt I made from a child's sleeping bag that I slip between layers and it makes a huge difference in R value.
I haven't found that the quilt gets compressed between layers enough to keep it from performing, which I would have thought to be the case.

I wear fleece and wool clothing to supplement my hammock insulation at night, which is nice if I have to get up and it's freezing out. I only have to slip on shoes and I'm good for a hour or so with the clothes I'm already wearing.

When it gets below freezing, I start using my down underquilt and a synthetic top quilt I made.

In the summer I do find the second layer helps stop mosquito bites, but i generally have been putting a 1/8" foam pad between layers for extra protection when they are bad.

For me the second layer helps by giving me a way to install insulation quickly and cheaply I guess, but no help by itself really.