R=.39 is for Durcok NexGen. Screw down the second layer too. The mortar between the Durock layers is not necessary.
Crap. After calling every tile store in town, and also Lowes and HD an hour away, none of them carried or had heard of "next gen" durock. I looked up the R value on the USG/Durock website and it listed the R value of 0.39 for "Durock brand cement board". I've read elsewhere that regular Durock is only R 0.2. Not sure where that came from though. Here's the product info from USG. R value listed on page 3 under "technical data".
http://www.usg.com/content/dam/USG_...durock-cement-board-system-guide-en-SA932.pdf.