Preheat the oven to 350 degrees. Roll out one pie crust and set in an ungreased 9-inch pie plate, cutting and rolling the crust to fit the edges. Roll out the second pie crust and shape as desired. Combine the sour cream, 2 egg yolks, and 4 tablespoons of sugar until blended. Spread over the bottom pie crust. Spoon the cherry pie filling over 2/3 of crust bottom, and the blueberry pie filling over 1/3 of the crust bottom. Decorate the top of the pie as desired with the second pie crust. Mix 1 egg yolk and the milk, and brush the top crust pieces. Bake on a baking sheet for 60-70 minutes or until the topping is set and the crust is browned. Check after 30 minutes and cover with foil if crust is turning too brown. Cool for about 1 hour before serving.