A simultaneously molded plurality of composite shingles or double walled shingles or other substantially planar exterior building products are provided, each being comprised of two or more layers of thermoplastic shingle material, such that a weatherable capstock material is provided outside a core material, in the form of a composite parison that is then blow molded in a hollow mold, to produce novel shingles. Ventilation and/or insulation can be provided to a hollow interior between spaced apart walls of a double walled shingle.