This simple tutorial demonstrates how to combine both normal maps and bump maps together in one Shader. If you add a normal map in Maya to a shader, it will by default, connect itself to the Bump attribute. This technique can also be used for combining two normal maps together.
The Maya scene is available here.
Bump map Normal map
To combine bump2d nodes you must connect the outNormal from the first bump2d node to the normalCamera of the second node. This can be done via the Connection Editor (make sure to enable 'Show Hidden' in order to see the normalCamera attribute).
The bump node has to be connected to the Standard Surface shader first and then you can add the normal map node onto it.
OutNormal from the first bump2d node is connected to the normalCamera of the second node
Bump map and normal map combined