Martha's sister-in-law Rita Christiansen never adds too much fruit to the filling because, when mixed with other fruit, rhubarb will absorb some of their flavors, overwhelming its own subtle taste.
Preheat oven to 350 degrees.
On a lightly floured surface, roll out one half of dough for bottom crust. Fit into an 11-inch pie plate. Trim pastry using scissors or a sharp paring knife, and chill. Keep remaining half of dough, which will be used to make the lattice, refrigerated and wrapped in plastic.
Combine rhubarb and raspberries in a large bowl. Sprinkle with sugar, flour, and lemon juice. Toss gently so as not to break raspberries. Spoon into pie shell, and dot with butter. Refrigerate while preparing lattice top.
In a small bowl, combine egg and heavy cream, whisking lightly; set aside. On a lightly floured surface, roll out remaining piece of dough for lattice, and cut it into 1-inch strips. Weave strips together over filling, and place an extra strip or two along the edge, where the bottom crust joins the lattice top; press this strip with tines of a fork to seal well. Brush lattice and all pastry edges with egg glaze, sprinkle top with sugar, and place pie on a parchment paper's lined baking sheet.
Bake until pastry is golden brown and juices in center of pie are bubbling, 1 1/4 to 1 1/2 hours. Transfer to a wire rack to cool before serving.