Copy of Carpaccio detail, acrylic on board - Art Sulger
Ok, I give up - I can't get a good tone out of a 12-string using the classical right hand position. It occurred to me that Lute players deal with lots of doubled strings. I'm trying it. Seems to offer more control over dynamics and timbre.
Here's my latest effort emphasizing good tone, tempo, and dynamics, the Allemande from a Bach Lute Suite, BWV 996.