You need to configure the client to mount both namespaces by specifying the appropriate properties in the core-site.xml.
Keep a note that HDFS Federation improves the existing HDFS architecture through a clear separation of namespace and storage, enabling generic block storage layer. It enables support for multiple namespaces in the cluster to improve scalability and isolation. Federation also opens up the architecture, expanding the applicability of HDFS cluster to new implementations and use cases.
Hope this helps!