This is a node group based upon various methods to combine normals described in this Stackexchange question: https://blender.stackexchange.com/questions/38298/how-to-combine-two-normal-maps
Credits: David, radcapricorn, Secrop, Stan Pancakes, Hellfireboy, Rico Cilliers, Matt Ferndz, Bithur, Miki
Thanks John, will check this out.