Bring rum with raisins to a boil in a 1-quart heavy saucepan, then remove from heat and let stand, covered, 1 hour.
NOTE: Raisins can be soaked in rum 1 day ahead, cooled completely, and kept in an airtight container at room temperature.
For Double-Crust Pie Dough:
Blend together flour, butter, shortening, and salt in a bowl with your fingertips or a pastry blender (or pulse in a food processor) just until mixture resembles coarse meal with some small (roughly pea-size) butter lumps.
Drizzle 5 tablespoons ice water evenly over mixture. Gently stir with a fork (or pulse) until incorporated.
Squeeze a small handful of dough: If it doesn't hold together, add more ice water, 1/2 tablespoon at a time, stirring (or pulsing) until incorporated. Do NOT overwork dough, or pastry will be tough.
Turn dough out onto a work surface. Divide dough into 8 portions.
With heel of your hand, smear each portion once or twice in a forward motion to help distribute fat. Gather all dough together with pastry scraper into one piece.
Divide dough with one half slightly larger, then form each into a ball and flatten each into a 5-inch disk. If dough is sticky, dust lightly with additional flour.
Wrap each disk in plastic wrap and chill until firm, at least 1 hour.
NOTE: Dough can be chilled up to 2 days ahead.
To Complete the Pie:
Put oven rack in middle position with a large heavy baking sheet on rack and preheat oven to 425°F.
Rub together brown sugar, flour, zest, cinnamon, nutmeg, and salt with your fingers in a large bowl until no lumps remain.
Peel and core apples, then cut into 1/2-inch-wide wedges and add to sugar mixture, tossing gently to coat.
Add raisins with any liquid and toss until combined.
Roll out larger piece of dough into a 13-inch round (keep remaining piece chilled) on a lightly floured surface with a lightly floured rolling pin. Fit into a 9-inch pie plate (4-cup capacity) and trim edge, leaving a 1/2-inch overhang.
**Chill shell while rolling out top crust.
Roll out smaller piece of dough on a lightly floured surface with lightly floured rolling pin into an 11-inch round.
Spoon filling evenly into shell, then dot top with butter.
Brush pastry overhang with some of milk, then cover pie with top pastry round.
Trim pastry edges, if needed/desired, to edge of pie plate (**NOTE: I like to leave a bit of extra dough on the edges and roll it over on itself before crimping).
Press edges together and crimp decoratively.
Lightly brush top of pie with some of remaining milk and sprinkle all over with sanding sugar. Cut 3 steam vents in top crust with a small sharp knife.
Bake pie on hot baking sheet 20 minutes. Reduce oven temperature to 375°F and continue to bake until crust is golden and filling is bubbling, 45 to 50 minutes more.
Cool pie on a rack to warm or room temperature, about 1 1/2 hours.
**NOTE: Pie can be made 8 hours ahead and kept, uncovered, at room temperature.