How to use DoubleTwist Sync with Android Marshmallow adoptable storage
Ok, I figured it out.  It wasn't some fundamental problem with how adoptable storage works, I just didn't do it right.  After setting up the SD card as adoptable storage, there is one more step which is not made clear by most tutorials.  You need to select the card and in the Options, select "Migrate Data" which completes the process and makes the card available for use by other apps, or something like that.