Combine flour, sugar, and salt. With a pastry blender, cut shortening and COLD butter into dry ingredients until mixture resembles pea-sized crumbs. Pour in 1/3 cup of water and continue blending until dough comes together. If it looks dry, add the other two tablespoons of water. Divide dough into two round balls, press flat, cover with plastic wrap, and refreigerate for at least 30 minutes.
To make filling:
In a bowl, mix cut rhubarb and strawberries, sugar, cornstarch, and salt. Let sit for 20 minutes so the fruits can absorb the ingredients.
To assemble pie:
Remove one half of the dough from refrigerator. Roll it out to about a 10-inch circle as best you can. Use a little flour if dough is too sticky. Work the dough as little as possible. Place the circle into a 9-inch pie dish. Press to the sides and trim off any excess. Remove the other half of your dough from the fridge and repeat. Roll it out to make another circle. With a pizza or pastry cutter, cut pieces of dough 1-inch wide, you'll need 6 of them for a nice lattice top. Pour fruit mixture into prepared pie dish. Alternate strips of dough to create a lattice top. Trim off excess. Brush the top of crust with milk.
Bake in oven on 425 for 30 minutes. Lower the temperature to 350 degrees, bake another 25-30 minutes until juices are bubbling and crust is golden brown.