Setup
SQL> show parameter control_files;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
control_files string +DATA/grac4/controlfile/current.260.826111693
SQL> alter system set control_files='+DATA/grac4/controlfile/current.260.826111693', '+FRA2' scope=spfile sid='*';
System altered.
Stop the database and start it in nomount mode.
[grid@grac41 Desktop]$ srvctl stop database -d grac4
[grid@grac41 Desktop]$ srvctl start database -d grac4 -o nomount
Create the control file using RMAN by pointing to an actual existing controlfile.
[oracle@grac41 RMAN]$ rman target=/
RMAN> restore controlfile from '+DATA/grac4/controlfile/current.260.826111693';
Starting restore at 11-JUL-14
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=34 instance=grac41 device type=DISK
channel ORA_DISK_1: copied control file copy
output file name=+DATA/grac4/controlfile/current.260.826111693
output file name=+FRA2/grac4/controlfile/current.321.852654927
Finished restore at 11-JUL-14
Stop the database and start it in normal mode.
[oracle@grac41 RMAN]$ srvctl stop database -d grac4
[oracle@grac41 RMAN]$ srvctl start database -d grac4
SQL> show parameter control_files;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
control_files string +DATA/grac4/controlfile/current.260.826111693,
+FRA2/grac4/controlfile/current.321.852654927
Verify that duplexed CF is known by FRA
SQL> select status, name, IS_RECOVERY_DEST_FILE is_rdf from v$controlfile;
STATUS NAME IS_RDF
------- -------------------------------------------------- ------
+DATA/grac4/controlfile/current.260.826111693 NO
+FRA2/grac4/controlfile/current.321.852654927 YES
Reference