if it was me doing my own repair on a two ply sail, I would very carefully fit a piece of 2" wide sail repair tape INSIDE the tear and get one half of the torn area to lay nicely on the tape, then carefully get the other side to lay down to match it perfectly....
this may be best done by making some index marks across the tear to both "halves" of the torn material before fitting the tape in, so you know where the material should line up while you have your hands in there holding it open so the tape dont stick in the wrong place....
once you have that layed down nicely, then put another piece of tape over the outside of it.... and one larger piece over all of that so it will grab material about 3inches either side of, and past the ends of the tear....
I wouldnt worry about stitching it...
if you wanted to stitch it, you only have two options.... separate the plies at the edge seam, and make the repair as normal... or dont seperate them and just stitch thru the second ply.... Myself, I would rather rely only on the sail tape alone rather than stitch the two plys together for a repair, as I think it will create a hard spot in the "working" of the fabric and cause more serious problems later....