A simple approach is to transform the normal maps into world coordinates with NormalMap nodes, mix both normals with an RGBmixer and normalize the output color with a VectorMath node. Note that this will just output an interpolation from one normal vector to the other.
If you need a more complex approach, you can use this node setup. This one will let you use a base normal map, and add details to it from a secondary normal map, using quaternion rotations. For some setups (like adding scratches to a bumped surface) this will give you better results.