We can enable flashback on RAC 11gr2 when one instance open, another mount.

I just try a test:

- Instance 1: Open Read - Write
- Instance 2: Open mount
- Do some commands in mount instance etc: flashback on ...

Instance 1:

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE

Instance 2:

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
MOUNTED

Views GV:

SQL> select open_mode from gv$database;

OPEN_MODE
--------------------
READ WRITE
MOUNTED

Do command enable flashback on instance 2, during instance 1 open read-write:

SQL> alter database flashback on;

Database altered.

View Alert log on intance 1:

RVWR started with pid=49, OS id=11480
Thu Apr 02 11:33:01 2015
Allocated 3981120 bytes in shared pool for flashback generation buffer
Thu Apr 02 11:33:02 2015
NOTE: dependency between database DBCORE and diskgroup resource ora.DISK3.dg is established
Thu Apr 02 12:46:13 2015


View Alert log on intance 2:

alter database flashback on
Thu Apr 02 11:32:46 2015
RVWR started with pid=33, OS id=7909
SUCCESS: diskgroup DISK3 was mounted
Allocated 3981120 bytes in shared pool for flashback generation buffer
Thu Apr 02 11:32:49 2015
NOTE: dependency between database DBCORE and diskgroup resource ora.DISK3.dg is established
Thu Apr 02 11:33:07 2015
Flashback Database Enabled at SCN 1031383
Completed: alter database flashback on

Open instance 2:

SQL> alter database open;


in summary: we can enable flashback on RAC 11gr2 when one instance open, another mount.