Glued double layer construction is now only reserved now for boards that need to be super strong (river / rapid) or cheeper brands using outdated technology. A glued double layer construction starts with the single layer board; dropstitch covered in an initial PVC layer. From there, a second layer of PVC is glued to the board (not laminated).
This differs to the stringer technology as it is a full coating over the whole board, not just a strip down the centre. It is also usual to have an extra layer around the rails of the board.
The extra PVC layer helps to increase the strength of the board. On the down-side the extra material makes these iSUPs heavier.