Oracle 12c backup question with FRA
Duplexing Backup Sets


  ... It is not possible to duplex backup sets to the fast recovery
  area.


A similar effect could be achieved by setting the format to the location of FRA, but it will not be the same, as these files will not be accounted in the space usage of FRA.

configure device type disk parallelism 2;
configure channel device type disk format   '/fra/%U',   '/oracle_backup/%U';
configure datafile backup copies for device type disk to 2;
configure archivelog backup copies for device type disk to 2;
configure default device type to disk;

backup database plus archivelog;