Special equipment: 9-inch ceramic or glass pie dish
Line a baking sheet with foil. Place the baking sheet on the center rack of the oven and preheat the oven to 400 degrees F. Allow the baking sheet to heat for 15 minutes.
To a large bowl, add the blackberries, raspberries, 1 cup of the sugar, 2 tablespoons of the cinnamon, the arrowroot, flour and salt, and toss gently to combine. In a separate small bowl mix the remaining 1 teaspoon sugar and 1/4 teaspoon cinnamon and set aside.
Butter a 9-inch ceramic or glass pie dish. Unroll one pie crust and place into the pie dish. Pour the berry mixture into the crust. Unroll the second pie crust and place on top of the filling. Pinch the edges of the top and bottom crusts together to seal. Fold the double edge under and crimp, making a decorative edge. Sprinkle the reserved sugar and cinnamon mixture over the top. Using a paring knife, cut a few slits in the top of the pie crust.
Place the pie into the oven on the prepared baking sheet and bake until the crust is deep golden and the filling is bubbling through the slits, 1 hour and 15 minutes.
Cool the pie to room temperature before serving.