For the crust: Place the flour, sugar, and salt in a food processor fitted with a metal blade; pulse just to combine the ingredients. Drop in the butter and shortening and pulse only until the butter and shortening are cut into the flour. Don’t overdo the mixing—what you're aiming for is to have some pieces the size of fat green peas and others the size of barley. Pulsing the machine on and off, gradually add the water until the dough looks evenly moistened and soft curds form.
To roll out the dough: Scrape the dough out of the bowl and onto a work surface. Gather together and divide in half, then form into two disks. Place a disk between two pieces of parchment paper and, with a rolling pin, roll it out, continually turning it ⅛ of a turn and rolling until the disk is just bigger than the pie pan. Place into the buttered pie pan and trim the edges. Make a second disk of crust using the same method; then place both in the refrigerator to chill.
For the filling: Place the sugar in a large bowl and grate the lemon zest directly over the sugar; combine the sugar and zest with your fingers. Add the peaches and blueberries and toss, then let sit for about 5 minutes. Taste the filling and add a squirt of lemon juice and more sugar if needed. Stir in a pinch of salt and flour and gently mix.
Preheat oven to 425 degrees F. Assemble the pie: Remove the pie shell and top crust from the refrigerator and pile the filling into the chilled pie shell. Place water around the edge of the shell, then lay the flat crust on top of the fruit. Squeeze the edges of the top and bottom together; use a fork to crimp the dough to seal it. Brush the crust with an egg wash. Sprinkle sugar on top, then use a knife to make steam vents in the dough, along with a hole in the center. Place pie on a parchment-lined baking sheet and bake 20-30 minutes; then check on the pie, lower the temperature to 375 degrees F, and bake another 30 minutes. (If the crust is browning too quickly, lay a piece of foil on top before returning to the oven.)
The pie is finished when the crust is a beautiful golden brown and the filling is bubbling up through the slits. Remove the pie from the oven and allow it to rest for 30 minutes before slicing.