I am not the smartest card in the deck so here goes.
If I have an ND 16 filter lens fitted to my Hero 7 Black (as in pic1),
and I add an ND 16 "fit over" filter, (pic 2) do I get "ND32"?
Like I said I am not the sharpest knife in the drawer.
I will go with a single ND32 as you suggest but at some point when I have a
few $$$$ to waste I will try the other way (just to compare)