@John Jackson I am not aware of any capabilities for multipule secondary namenodes. If that would be possible then I assume enhancements to the journal keeper service will need to be made to keep multipule instances in sync. Would like to see others opinion if that is possible and supported. To the best of my knowledge it is not. One possibility you have is HDFS federation. where you can have multipule name nodes and each once having secondary name node. Each namenode would be responsible for slice of your namespace. here is the architecture
In order to scale the name service horizontally, federation uses multiple independent Namenodes/namespaces. The Namenodes are federated; the Namenodes are independent and do not require coordination with each other. The Datanodes are used as common storage for blocks by all the Namenodes. Each Datanode registers with all the Namenodes in the cluster. Datanodes send periodic heartbeats and block reports. They also handle commands from the Namenodes.
Users may use ViewFs to create personalized namespace views. ViewFs is analogous to client side mount tables in some Unix/Linux systems.