The difference between the SAV environment and SEP environment in this case needs to be first considered:
In SAV environment,the Reporting server is a separate administrator tool which is used to pull the reports from the Symantec System Centre.
So when the reporting server is configured in the Symantec system centre you will be able to pull the reports of the SAV servers and their respective clients which are in server groups.
In SEP Environment,there is no seperate Reporting server administrator tool,it is all integrated in the Symantec Endpoint Manager,to pull the reports,logs or configure email alerts.
So if you are migrating from SAV to SEP environment,the clients groups and clients information(reports) can be pulled from the SEPM itself.There is no concept of secondary server in SEP,only 1 server which is SEPM and SEP clients.But you can build GUP's in the branch locations so that the clients in that locations can get definitions from their respective GUP(Group update providers).
The centralized SEPM can be used to the pull all the reports of the GUP's and the SEP clients.