Both instances of Edge SSO must be connected to the same Postgres server. Apigee recommends using a dedicated Postgres server for Edge SSO and not use the same Postgres server that you installed with Edge.
Whats the purpose of Edge SSO communication to Postgres Master ?
Understand that Edge SSO communicates to OpenLDAP for User Management information, but reason for EDGEE SSO with Postgres Master is not clear in the documentation
When you install the sso, it creates a table for sso in Postgres. The table contains the token and user-related information. You cannot directly write to the standby Postgres, it always writes to the Postgres master.