Preheat the oven to 375. Place one of the pie crusts into the bottom of a deep-dish pie plate. Set aside. Combine rhubarb, blueberries, 1/2 cup sugar, cornstarch, lemon juice, and cinnamon in a large mixing bowl. Stir until all is combined. Pour mixture over the bottom crust. Spread mixture out until level. Dot mixture with butter. Place top crust over the pie crimping under the bottom crust as you go. Brush the top crust with egg white and sprinkle a teaspoon of sugar over it. Bake for 40-45 minutes or until pie crust is golden brown and filling is bubbling. Remove from the oven and allow to cool completely.