From what I have read and understood from their documentation is that only single layer IoT hierarchies can be maintained with either of their gateways. That is a field gateway(sort of like a smart router) sits between the server and edge devices and does the preprocessing and edge computing.
What I am wondering is whether field gateways provided from either vendor(AWS or Azure) can be nested as parent and children to create multi-layered IoT device hierarchies. That is gateways connected to gateways and so on.
EDIT - This kind of hierarchies would create Fog networks which would enable sub-networks within the hierarchy to function more independently without being over-reliant on the server. Also, they'd reduce the load on the server if the edge gateway could do edge computing while reducing latency as well.
Currently the edge device only able to communicate with Azure IoT Hub directly. It doesn't support nest multiple Azure IoT Edge gateways.
However, this capability is planning on adding to the product. This feature will be considered in the future but unfortunately there is no timeline yet.