As RMAN needs a consistent view of the control file it takes a backup of the controlfile by creating a snapshot and during the backup RMAN uses the snapshot of the controlfile. By default the snapshot controlfile is created in $ORACLE_HOME/dbs/snapcf_ORACLE_SID.f which can be changed by setting the following parameter in RMAN

$ rman target /
— display parameter value
RMAN> show all;
..
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/u01/10.2.0.4/TESTDB/snapcf_TESTDB.f'; # default
— set to new path for controlfile snapshot
RMAN> CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/u02/TESTDB/snapcf_TESTDB.f';

When multiple RMAN are running one could run into an issue RMAN-08512 where RMAN is waiting for getting a lock on snapshot controlfile header. To find the session run the following SQL.

SELECT vs.sid, vs.username, vs.program, vs.module, TO_CHAR(vs.logon_time, 'DD-MON-YYYY HH24:MI:SS')
FROM v$session vs, v$enqueue_lock vel
WHERE vs.sid = vel.sid AND vel.type = 'CF' AND vel.id1 = 0 AND vel.id2 = 2