RMAN backup of RAC database failing
Are you able to tnsping realdb_1, realdb_2 individually? 

If you're using SCAN, but haven't manually added node specific entries to your tnsnames.ora, this problem is likely to occur upon requesting connection to a specific node, since SCAN may be configured to resolve to the service_name of the RAC, rather than to the "instance_name".

Here's my tnsnames.ora:

USAPROJ =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac-scan.localdomain)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = usaproj.localdomain)
    )
  )

USAPROJ1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac-scan.localdomain)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = usaproj.localdomain)
      (INSTANCE_NAME = usaproj1)   &lt;-------
        )
  )


I created another entry, usaproj1, and in addition to service_name parameter, I added "instance_name", to specify the node I wanted to connect. 

Hope it helps.