In a two-server dataguard setup, can I store the recovery catalog in the same database?
If I correctly understand your question as: "I want to create a standby database and later on take backups on the primary like if there would be no standby present"

In this case, you do not need a recovery catalog: quote "An RMAN recovery catalog is required so that backups taken on one database server can be restored to another database server. It is not sufficient to use only the control file as the RMAN repository because the primary database will have no knowledge of backups taken on the standby database." See http://download.oracle.com/docs/cd/E11882_01/server.112/e25608/rman.htm#BAJEDHFB