Preheat oven to 375°F.
To a medium saucepan over medium heat, add 1 cup of rhubarb and 1 cup of strawberries. Cook, stirring occasionally, until the rhubarb becomes soft, the strawberries start to break down, and the mixture has reduced to a jam-like consistency, about 5 minutes. Remove from heat and allow to cool slightly.
Once slightly cooled, add the remaining uncooked rhubarb, strawberries, sugar, and cornstarch. Stir to mix well. Set aside.
On a lightly floured surface, roll out one of the pie crusts into a 12-inch circle. Place into a 9-inch pie pan, overlapping the sides.
Pour the berry mixture into the prepared pie crust. Add the diced butter evenly over the top of the filling.
Roll out the second pie crust into a 12-inch circle and place it over the filled pie. Seal the edges and trim excess dough. Using your fingers or a fork, crimp the edges of the two pie crusts together.
Brush the top crust with the beaten egg. Evenly sprinkle the turbinado sugar on top of the egg wash, if using.
Using a sharp knife, cut several small slits into the pie crust for venting.
Bake for 60-65 minutes or until the filling is bubbling and the crust is golden brown. (If the crust is getting too dark, cover with a pie shield or aluminum foil.)
Allow the pie to cool completely before cutting and serving.