Preheat the oven to 450 degrees F (230 degrees C). Line 2 pie plates with the pie crusts.
Stir sour cream and eggs together in a large bowl. Stir in sugar, 4 tablespoons flour, vanilla extract, and salt. Stir in strawberries and rhubarb. Pour filling into the 2 pie crusts. Cover with aluminum foil.
Bake in the preheated oven until crust is lightly browned, about 15 minutes. Reduce heat to 350 degrees F (175 degrees C); continue baking until filling is soft and nicely thickened, about 20 minutes more. Leave oven on.
Stir brown sugar and 1/2 cup flour together in a bowl. Cut in butter until mixture resembles coarse crumbs. Sprinkle mixture over the pie fillings.
Return pies to the hot oven. Bake until filling is set, 20 to 25 minutes.