In your pipeline you want to have something like Xnormal or nDo already running, but for those times when you just want to quickly go in and make a normal map stronger this could be excatly what you need.
These are the few steps you need to take:
1 – duplicate your normal map twice (copy 1 and copy 2 in the image)
2 – open the advanced blending options by double clicking your layer (click to the right of the name “copy 1” so you don’t enter re-name mode).
4 – on the second layer (marked copy 2) turn on the red and green channels and turn of the blue channel. Set the blend mode to overlay.
And that’s all there is to it. If you want to make it even stronger, just group and create another set of the two layers you just created on top of everything.