Preheat the oven to 400 degrees. Cut the rhubarb into one-half-inch thick pieces (it should total about three and one-half cups). Toss the rhubarb and the strawberries with the sugars, cornstarch, spices and salt to coat evenly. Arrange one piecrust in a 9-inch glass pie dish and pour in the filling, spreading evenly. Trim the bottom crust and prepare the top crust as desired. If using a solid top crust, pierce the top with decorative slits. Whisk egg yolk and water together to form a glaze and brush it over the crust. Cover the crust edges with foil to prevent burning. Transfer the pie to a baking sheet to catch any drips in the oven and bake for 20 minutes. Reduce the oven temperature to 350 degrees and bake another 80 to 90 minutes, until the crust is golden brown and flaky, and the filling thickens. Remove the foil from the edges for the last 15 to 20 minutes of baking. Cool on a rack so the filling sets before serving.