Preheat the oven to 400°F.
Roll out one pie crust and place it into a 9-inch pie plate. Chill while preparing the filling.
In a large bowl, combine the chopped rhubarb and sliced strawberries.
Add the brown sugar, granulated sugar, cornstarch, vanilla, and orange juice. Stir gently until the fruit is evenly coated.
Spoon the filling into the prepared pie crust, scraping any sugar and cornstarch mixture from the bowl into the pie. Dot the top of the filling with 2 tablespoons of butter cut into small pieces.
Roll out the second pie crust and place it over the filling, or cut it into strips for a lattice top.
Trim and crimp the edges. Cut vents in the top crust if using a full top crust.
If desired, brush the crust lightly with egg wash and sprinkle with coarse sugar.
Place the pie on a baking sheet.
Bake at 400°F for 20 minutes.
Reduce the oven temperature to 350°F and bake for another 35 to 45 minutes, or until the crust is golden brown and the filling is bubbling.
Let the pie cool for at least 3 to 4 hours before slicing.
Serve plain, with whipped cream, or with vanilla ice cream.