During the installation the session server and MSS have already exchanged and trusted their certificates, all MSS servers already trust all existing session servers. However, when you add more session servers, a trust relationship needs to be established between the new session servers and the existing MSS servers
To add more session servers
Import the MSS server certificate into the session server truststore. See Import a certificate into the session server’s truststore.
Import the session server certificate into the MSS server truststore. See General Security and Certificates in the MSS documentation.
Retrieve the service.registry.password from the MSS server container.properties file.
Set the service.registry.password in the session server container.properties file.