Sails with taffeta on both sides can be made in two ways. Some sailcloth styles are made with taffeta on one side and exposed mylar on the other. UK Sailmakers uses this cloth for performance cruising Tape-Drive or X-Drive sails. The tapes are adhered to the mylar side and then a layer of taffeta is applied over the tapes. The taffeta does not add any strength to the sail, but it gives the sail abrasion resistance. It also makes the sail look like is made from a woven material instead of a laminate. Taffeta layers add a bit of weight; thus, the reason flat-out racers do not choose this durability-adding option.
A second option is to have the taffeta added over both sides of a Titanium sail. Shown below, this combination makes the highest performance cruising sail.