It's ok in A6.3: under bump->texture corona_normal; under corona_normal->c4d_layer; in c4d_layer i've used 2 corona_normal shaders mixed 50%. Result: multi normal shader.
Nesting system is incredibly powerfull: try to insert a filter shader under c4d_layer and under filter use a corona_normal. You have a perfect control of the normal map. If you select "different" under c4d_layer 1st shader you can blend the normal textures and things start to became interesting.