Roll one dough disk out into a 12-inch round on a lightly floured surface. Gently transfer dough round to a 9-inch deep dish pie plate. Gently ease and press dough onto bottom and up sides, letting excess hang over side. Loosely wrap dough-lined pie plate with plastic and chill until dough is firm, about 20 minutes.
Roll second dough disk out into a 12-inch round on a lightly floured surface. Punch out ½–1 -inch holes (reserving rounds for decorating, if desired), or cut into 1-inch strips; transfer dough round to a parchment lined baking sheet, cover with plastic wrap and chill 20 minutes.
Combine blueberries, rhubarb, sugar, vanilla, lemon zest, lemon juice, salt, nutmeg, and cardamom in a large bowl. Add tapioca and toss to combine.
Transfer filling to chilled dough-lined pie plate; sprinkle butter evenly over filling.
Gently place remaining dough round over top of filling (or create a lattice top). Trim overhang to ½-inch then tuck edges under and crimp using thumbs and pointer fingers.
Brush dough with egg wash; transfer pie to freezer and chill until dough is firm, about 45 minutes (skip this step if using frozen fruit).
Preheat oven to 400°F with rack set in lowest position.
Transfer pie to a baking sheet and bake until crust is light golden brown, 25 minutes.
Reduce oven temperature to 350°F, rotate baking sheet, and continue to bake until crust is golden brown, 30 minutes.
Sprinkle sugar over top of crust and bake until crust is deep golden brown and filling is bubbling, about 30 more minutes.
Transfer pie to a wire rack; let cool completely, at least 2 hours.