Heat oven to 400°F. Make pie crusts as directed on box for Two-Crust Pie using 9-inch glass pie plate.
In large bowl, place reserved strawberry juice. In small bowl, mix granulated sugar and cornstarch; whisk into reserved strawberry juice. Stir in rhubarb and strawberries; spoon into crust-lined pie plate. Dot with butter.
To make lattice top, cut second crust into 1/2-inch-wide strips with pastry cutter. Place half of the strips across filling in pie plate. Weave remaining strips with first strips to form lattice. Trim ends of strips even with edge of bottom crust. Fold trimmed edge of bottom crust over ends of strips, forming a high stand-up rim. Seal and flute. In small bowl, mix egg yolk and water. Brush lattice strips with egg yolk mixture; sprinkle with sugar.
Bake 20 minutes; cover edge of crust with strips of foil to prevent excessive browning. Bake 20 to 30 minutes longer or until golden brown. Cool at least 2 hours before serving.