Can't seem to find the recent threads on this topic, so here is the update on trying layers of insultex placed between the layers of the BB dbl.
First test, a single layer of IX did not keep me warm at all. Tried 3 layers as suggested last night- it was a wee bit warmer, but still ended up with a chilly backside.
I've concluded that you really need a little more airspace with IX- Thats why it seems to work best as an UQ. However, I have a old DYI fleece sleeping bag (top folds over and snaps- no zippers) with a waterproof nylon back. Its pretty thick and the nylon is stiff enough that the whole thing keeps its shape and could work well in this situation between layers of the double. Ill try that and add insultex. Probably dont need the insultex as the fleece is plenty warm, but could increase warmth if it was cooler out... Granted that set up will only be for car/backyard camping due to its bulkiness and weight... (Unless Arlo and Spike pack it for ya on the trail!)