In further considering the two-ply approach, I've been researching the two-ply wrap around sails
originally used on Freedom yachts. While not identical to two layers fully sewn together, there are useful similarities.
I found it interesting that I was unable to find any discussion about either chafe or mildew being a problem, and the key reasons for moving later to single
ply with a track was to allow for battons and because the wrap around luff had a tendency to creep around the mast
and spoil the sail shape.
I also found reference to a few offshore racing
yachts of yore which had fully two-ply sails
for strength, in the early days of dacron.
So I'm still not ruling out the two ply approach. Will be talking more with sailmakers to get additional expert input on the options.