Instructions

 Line rimmed baking sheet with parchment paper. Roll each dough square into 16 by 11-inch rectangle on floured counter; stack on prepared sheet, separated by second sheet of parchment. Cover loosely with plastic wrap and refrigerate until dough is firm but still pliable, about 10 minutes.
 Using parchment as sling, transfer chilled dough rectangles to counter; discard parchment. Wipe sheet clean with paper towels and spray with vegetable oil spray. Starting at short side of 1 dough rectangle, loosely roll around rolling pin, then gently unroll over half of long side of prepared sheet, leaving about 2 inches of dough overhanging 3 edges. Repeat with second dough rectangle, unrolling it over empty side of sheet and overlapping first dough piece by ½ inch.
 Ease dough into sheet by gently lifting edges of dough with your hand while pressing into sheet bottom with your other hand. Brush overlapping edge of dough rectangles with water and press to seal. Leave any dough that overhangs sheet in place. Cover loosely with plastic and refrigerate until firm, about 30 minutes.
 Adjust oven racks to lower-middle and lowest positions and heat oven to 375 degrees. Gently toss nectarines, raspberries, ½ cup sugar, and salt together in bowl. Spread nectarine mixture evenly over chilled dough-lined sheet. Fold overhanging dough over filling, pleating corners, trimming excess dough as needed, and pinching overlapping edges to secure. (If dough is too stiff to fold, let stand at room temperature until pliable.) Brush dough with water and sprinkle evenly with remaining 1 tablespoon sugar.
 Place large sheet of aluminum foil directly on lower rack (to catch any bubbling juices). Place galette on upper rack and bake until crust is deep golden brown and fruit is bubbling, about 1 hour, rotating sheet halfway through baking. Let galette cool on wire rack until filling has set, about 2 hours. Serve.