can we register both Primary & Standby Databases with RMAN recovery catalog?
The primary and standby databases in a Data Guard system have the same DBID and database name. Each database in the Data Guard environment must have a separate DB UNIQUE NAME value to be eligible for registration in the recovery catalogue.
RMAN in a Data Guard environment, you can only register the primary database with the REGISTER DATABASE command. RMAN automatically registers a standby database in the recovery catalogue when you connect to it as TARGET.
Read the link mentioned below for the further more details: