In a food processor, mix the flour, sugar, and salt.
Add the butter that's been cut into pieces and the shortening. Pulse in 5-6 one-second increments until the butter and shortening are no larger than the size of a pea.
With the processor running, add enough water for the dough to form a ball. Immediately stop the processor and dump the dough on a sheet of plastic wrap.
Using plastic wrap, knead into a cohesive dough. Wrap tightly and pat into a disc.
Chill for 12-15 minutes before rolling out to relax the dough.
After the dough has rested, remove from plastic wrap and roll out on a well-floured surface. Roll out to a 12-inch circle for a 9-inch pie dish.
Lay dough in the pie dish, allowing the dough to fill the corners with the excess coming up over the rim of the dish.
For the filling
Preheat oven to 400°F.
In a large bowl, combine the rhubarb, blueberries, salt, minute tapioca, sugar, and lemon juice. Stir to combine and pour into a rolled out crust fitted in a 9-inch pie pan.
Fill the crust with the filling and dot the top with the butter.
Brush the edges of the filled pie crust with water.
Roll out a second crust to 12 inches and place over filling and dots of butter.
Trim the edges of the crusts, leaving a one-inch overhang. Roll edges under crust and press slightly to adhere. Crimp edges.
Whisk the egg and water together until smooth. Brush top of the crust with the whisked egg and water. Sprinkle with sugar.
Cut at least two vents into top crust.
Place pie on a parchment-lined baking dish and place in oven. Bake 20 minutes at 400°F then turn the oven down to 375°F until the top and bottom crust are golden brown and the center of the pie filling is bubbling, 45-60 minutes. If edges of crust are browning too quickly, cover with a crust cover or with aluminum foil.
Remove from oven and allow to cool to room temperature before cutting, about 4 hours.