Remove both rectangles of chilled dough from the refrigerator.
Butter an 11 1/2- by 16-inch sheet pan with a 1-inch rim, then line with parchment paper.
On a lightly floured work surface, unwrap 1 rectangle of dough and roll into a 16- by 20-inch rectangle. Fit the dough inside the sheet pan, letting the edges overhang.
Roll the second dough rectangle into a 13- by 17-inch rectangle. Place on a second sheet of parchment paper, then lay that on top of the bottom crust and place in the refrigerator to chill while preparing the filling.
Combine rhubarb and blueberries in a large bowl, crushing about half of the blueberries with your hands. Peel and grate the apple, then add to the mixture, along with both sugars, allspice, cinnamon, cardamom, cloves, salt, cornstarch, lemon juice and bitters; mix thoroughly.
Remove the dough from the refrigerator and set the top crust aside. Pour filling into the sheet pan. Position the top crust rectangle squarely over the filling, then roll and pinch the excess crust inward to create an edge all the way around the pan. Crimp this edge as desired. Create several steam vents in a decorative pattern using a sharp paring knife.