Sometimes it can be useful to combine together multiple normal maps. Cycles offers an easy way to overlay bump mapping on top of an existing normal map, but there doesn’t appear to be a good solution when it comes to normal maps.

This is a little node setup that I’ve made that does a pretty decent job at that without messing up the scaling of the R/G/B components like MixRGB->Overlay would.

Plug your normal maps in the input, and get the combined version on the output: simple and easy.