Combine sugar, tapioca, and salt. Add to peaches and blueberries. Stir gently to mix well.
Sprinkle lemon juice over fruit. Toss lightly to mix.
Cut butter in small pieces. Place over fruit. Put bottom crust in pan, trim edges, and motion top edge. Place top crust over pie, trim edges, and flute. Cut slits in top curst in several places.
Bake 450 degrees for 10 minutes. Reduce to 350 degrees and bake for 30-45 minutes, or until lightly browned.