HH SS on WWBB
I have a bunch of hammocks and to name a few are the HHULBA, HHEXP and the WBBB 1.1 Dbl. I could have easily gotten away with a 1.0 Sgl but I enjoy the double bottom when using a CCF. IMO the interior space of the WBBB is pretty comparible to the Expedition. However, the footbox of the BB gives the hanger that much more room to stretch out.
To answer your question about whether the HH underpad will fit the BB Dbl my answer is yes. About a month ago I used the SS that goes with my HHULBA as a wind barrier and Yeti, CCF snugger-upper of sorts with the BB. I was amazed at the results. On a previous trip I didn't use the SS and I just couldn't get the Yeti to seal up the way that I needed it to and was left pretty darn cold all night with intermittent warmth and sleep. On my next trip out I decided to try the SS and it made all the difference in the world. The tempurature hovered at 27-29F and I was pretty darn comfortable. I believe that if I had doors on my Superfly it would have been perfect. I don't have a overcover so I can't speak for the fit.