For the crust:
In medium bowl, combine flour, salt and sugar. Add cubed butter and use pastry blender to cut butter into dry ingredients until size of small peas. Add water 1 tablespoon at a time until dough starts to come together into ball. Dump dough out onto clean surface and knead a few times to incorporate all of dry bits. Work quickly, do not over handle.
Divide dough in half and shape into 2 disks. Cover with plastic wrap and refrigerate for at least 2 hours.
For the pie:
Preheat oven to 400°F.
On a lightly floured surface, roll out one disk of pie crust into a rough 12-inch circle and transfer to a 9-inch pie plate.
In a large bowl toss the plums, raspberries, sugar and cornstarch. Dump into the crust-lined pie plate.
Roll out the second pie crust and place it on top of the pie, or cut into strips and weave a lattice. Trim edges and crimp as desired.
Brush pie crust with egg wash, if using, and place on a sheet pan lined with parchment paper. Bake until pie crust is golden brown and filling is bubbling, about 1 hour. If crust browns too quickly, cover with foil for the remaining baking time.