Mix the sweet cherries, rhubarb, granulated sugar, and tapioca in a large bowl.
Stir in the lemon juice and salt. Let sit for 15 minutes in order for the tapioca to thicken the filling.
While the filling is sitting, preheat the oven to 400°F (200°C). Line a pie plate with pie crust and trim to ½-inch over the edge. Place the crust in the freezer to stay cold until the filling is ready (no more than 15 minutes or your crust may start to freeze).
Pour the cherry rhubarb filling into the crust. Use your second pie crust to form a lattice on the top of the pie, or place the whole crust on top of the pie, cutting 3 slits to allow the steam to escape. Press the edges of the crust together to seal.
Bake in the oven 20 minutes, then reduce the temperature to 350°F (177°C) for for an additional 40-50 minutes, or until the crust is golden brown and the filling is bubbling. Remove from the oven and cool for at least 1 hour, allowing the filling to set.
An equal amount of cornstarch can be used in place of the tapioca. Stir the cornstarch into the sugar before mixing with the fruit, to prevent lumps.
You can use orange juice instead of lemon juice if you do not have lemons on hand.
If using frozen fruit, allow it to thaw, then drain it before mixing the filling.
Place a pie shield over the crust after 30 minutes of baking to help keep it from burning.
For a glossy crust, brush the top with an egg wash mixture (1 egg with 2 tablespoons of water or milk) before baking.
Store at room temperature for 2 days, or in the refrigerator for up to 4 days.