Prepare the pastry and filling:

1. Prepare the pie pastry and let it rest overnight, if possible.
2. Preheat the oven to 400ºF and set an oven rack in the lower third.
3. Roll out 1 prepared pastry disk to an 11-inch circle. Fit the pastry into a 9-inch pie plate, making sure to press the pastry into the sides and the bottom. Place in the refrigerator until ready to use.
4. In a large bowl, combine the nectarines, plums, sugar, quick-cooking tapioca, orange juice and zest, allspice, and salt. Carefully toss to coat. Let the fruit macerate and the tapioca soften for 10 minutes.

Make the crumble topping:

1. In a medium bowl, whisk together the melted butter, granulated sugar, brown sugar, cinnamon, and salt. Stir in the flour and oats until just combined. 
2. Place the filling and all the juices inside of the pie plate fitted with pastry. Arrange the fruit in the pie crust, eliminating any gaps, and lightly press down on the mixture to compact it.
3. Sprinkle the crumb topping evenly over the fruit.
4. Bake the pie on a rimmed baking sheet on the lowest rack for 25 minutes.
5. Loosely cover the pie with foil to prevent the crumble from getting too dark.
6. Bake until the juices are bubbling and the temperature of the middle of the pie reaches above 200ºF, about 35 to 40 more minutes.
7. Let the pie cool at room temperature for a minimum of 2 hours to let the juices set up.
8. Serve at room temperature.