Preheat oven to 425°.
Line a baking sheet with foil or parchment paper. Roll 1 dough portion into a 12-inch circle; place on foil. Combine peaches, blueberries, and 1/4 cup sugar. Arrange half of peach mixture in center of dough, leaving a 3-inch border. Fold edges of dough toward center, pressing gently to seal (dough will only partially cover peach mixture). Brush half of melted preserves over peach mixture and edges of dough.
Bake at 425° for 10 minutes. Reduce oven temperature to 350° (do not remove galette from oven); bake an additional 20 minutes or until lightly browned. Repeat procedure with remaining dough, peach mixture, and preserves. Sprinkle with 1 tablespoon of sugar. Serve warm or at room temperature. Cut each galette into 8 wedges.