Preheat oven to 425 degrees F (220 degrees C). Place one pie crust on the bottom of a pie plate.
Toss peach slices and blueberries in a large bowl with lemon juice. Combine sugar, tapioca, and salt in a small bowl; pour over peach mixture and toss to coat. Transfer to prepared pie plate and scatter butter pieces over fruit. Cover pie with remaining crust; press and seal the top and bottom crusts.
Cut several slits in the top crust to allow steam to escape, then brush with egg yolk.
Bake in preheated oven until golden brown, 45 to 50 minutes.
