To get another manager set up, you need the server.xml and the certificate (.ssl) files from the current manager. Then, run a Tomcat-only installation of the manager, so it does not try and create another database. The installer will ask you for the server.xml file and certificate files. It will also ask you for the xml files for the connection to the UMC and other data center things, which you can leave blank if you are not using them.
To push out new mangement server list, use the Diagnostic Detection policies (win an unix), and select "Advanced Agent Settings" and edit the section called "Modify the management server listused by the agent". Then, apply that policy to the agents.
There is a failover built into the agents, where if they cannot connect to the first management server in the list they will move onto the next one. It will attempt to fail back to the first one on the list every 60 min.
Note that when you add more servers, any bulk log files or Collect Agent Info's will be sent to the manager they are currently connected to, so if you cannot find a GAI or logs, you will have to check all your managers.