Stir together the flour, sugar, and salt. Using a pastry cutter or a food processor, cut the shortening and butter into the flour until the mixture resembles coarse crumbs. It is only about 10 pulses with a food processor but a bit more work by hand. One trick to make the process a little easier is to freeze your butter beforehand, then grate it into the flour mixture before cutting it in with the pastry cutter if not using a food processor.

Sprinkle 2 tablespoons of the ice water over the mixture. Using a fork, gently stir the water in to the flour mixture, then repeat with the remaining water, adding only 2 tablespoons at a time, just until the dough starts to come together.

Divide the dough into two even pieces. Turn each piece of dough onto a sheet of plastic wrap and flatten each into a disc and wrap tightly in plastic wrap. Refrigerate for 1 hour, then allow the chilled dough to sit on the counter to soften slightly for 10 minutes before rolling out.

Roll out the bottom crust. Gently line a pie plate with bottom crust and brush with part of the egg wash before adding filling.

To prepare filling, in a large bowl, add the raspberries, blackberries, sugar, cornstarch and lemon juice and gently toss to combine. Pour filling into the prepared pie crust.

Roll out top pie crust and gently lay the crust on top of the blackberry filling. Trim edges of the pie dough and crimp in a decorative pattern and cut slits in the top for venting. If desired, you could do a lattice crust or other decorative top. Brush the top crust with remaining egg wash and sprinkle with 1-2 tablespoons of sugar.

Bake for 45-60 minutes, until the filling is thick and bubbly and the crust is golden brown. If the edges of the crust begin to get too dark too soon, cover them with tin foil or use a pie guard so that they don't burn while the rest of the pie continues to bake.

When the pie is done, remove from the oven and allow the pie to cool for at least 3-4 hours before serving so the filling can set up.