Houdini 16 comes with new operators for normal map manipulation. The Normal Combine COP blends the incoming normals by just taking the average, and that is far from an optimal way to blend two normals. Stephen Hill has a very good web page on normal blending, and I have used his Reoriented Normal Mapping with great success in the past. The idea is, that instead of blending the normals you rotate one normal according to the orientation of the other normal. The end result is a much more natural blend. Here is a hip file with an RNM replacement for Normal Combine COP, and it also has a blend amount slider.