To make the crust: Line a 9" pie pan (or two 6" pie pans) with half the crust. Refrigerate while you make the filling, for at least 30 minutes., To make the filling: Whisk together the sugar, thickener, and salt., Toss the rhubarb and strawberries with the sugar mixture. Spoon the fruit into the pan(s), filling them about 3/4 full and mounding the filling a bit in the center., Place dabs of the butter atop the filling. Return the pie to the refrigerator., Preheat the oven to 425°F., Roll out the remaining crust, and cut it into star shapes., Place the pastry stars atop the filling. Brush with water and sprinkle with coarse white sparkling sugar., Bake the pie(s) for 30 minutes (for the 9" pie), or 20 minutes (for the 6" pies), then reduce the oven heat to 375°F and bake for an additional 30 to 40 minutes (9" pie), or 20 to 25 minutes (6" pies), until the filling is bubbling and the crust nicely browned., Remove the pie(s) from the oven, and let them cool for an hour or so before serving., The pie may be served warm, but it'll be a bit messy; it sets as it cools. Top with vanilla ice cream or whipped cream, if desired.