how to join two normal map textures in blender nodes
Normal Map Textures can be combined with this :
https://blog.selfshadow.com/publications/blending-in-detail/
The last node is a simple N*0.5+0.5
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You can combine your two images with a single Color Mix node
Uhmmm… I don’t think that a simple mix is the right way to mix two normal maps.
That blog page explain the reason.
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Yes… but you can use overlay for a quick check and if it looks good enough … then it looks good. 
As long as you know this is a quick hack and there might be a better way but you do not find the link/notice for the perfect solution yet. 