ORA-00304: requested INSTANCE_NUMBER is busy

Environment: 2 nodes Oracle RAC 11gR2, Linux 5
Scenario: Node1 has a single instance database named testdb, same testdb was restored to the Node2 (as single also) , while starting the instance on node2 for testdb (single instance) gave the error below:
ORA-00304: requested INSTANCE_NUMBER is busy

It was found it was due to the cluster services running although machines were different. To be sure this cluster services were stopped and the instance on node2 was running fine.
Requirement: To run the both instances (same name testdb) on both nodes at the same time while cluster services are still running.

Solution: Just change the db_unique_name parameter for one of the both DBs, and restart the instance.
SQL> show parameter unique

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unique_name                       string      testdb

SQL> alter system set db_unique_name=testdb2 scope=spfile;

System altered.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup;
ORACLE instance started.
Total System Global Area 1023004672 bytes
Fixed Size                  2219752 bytes
Variable Size             620757272 bytes
Database Buffers          394264576 bytes
Redo Buffers                5763072 bytes
Database mounted.
Database opened.

SQL> show parameter unique
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unique_name                       string      TESTDB2
SQL>