Preheat oven to 425 degrees F (220 degrees C). Place one pie crust into a pie plate.
Toss peaches and blueberries in a bowl with lemon juice. Mix sugar, tapioca, and salt in a small bowl and add to peach mixture; stir to combine. Allow fruit to rest for 15 minutes, then transfer to prepared pie plate. Scatter margarine pieces over fruit and brush the edge of the pie with water. Cover pie with remaining crust; press and seal the top and bottom crusts.
Bake in preheated oven until crust is golden brown and fruit is bubbly 45 to 50 minutes.