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.