Laminar morphology of high-density polyethylene (HDPE)/polyamide-6 (PA-6) blends was obtained, for the first time, in a co-rotating twin screw extruder. The morphology analysis in the interior of the adapter, which connects the extruder with a slit die, revealed that, under specific processing conditions, it is possible to produce layers of PA-6 in the end of the screw zone of the extruder. The micrographs of the extruded final products showed distributed layers across the sample thickness and along the flow direction. The effect of design and processing variables such as adapter angle, die gap, temperature profile, feed rate, and screw speed on the final product morphology were evaluated. In this research, blends were obtained with toluene permeability reduced by up to 113 times from the permeability of the matrix alone. This value is 2.5 times better than the best value reported for products of single screw extrusion. Mechanical tests showed that the HDPE/PA-6 extrudates maintain stiffness and tensile strength values comparable to those of polyethylene. In order to obtain products with optimum balance between the permeability and the mechanical properties, a statistical analysis was carried out. It produced useful information regarding the important processing and design factors that affect the product properties.