Connect to the same database instance with the same SID. The two instances are on different servers, such as my tnsnames file # tnsnames. ora Network Configuration File: D: \ oracle \ product \ 10.2.0 \ db_1 \ NETWORK \ ADMIN \ tnsnames. ora # Generated by Oracle configuration tools. DEV = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP) (HOST = 10.4.247.92) (PORT = 1521) (CONNECT_DATA = (SERVICE_NAME = DEV ))) ORCL = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP) (HOST = 12 7.0.0.1) (PORT = 1521) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = orcl) EXTPROC_CONNECTION_DATA = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = IPC) (KEY = EXTPROC1) (CONNECT_DATA = (SID = PLSExtProc) (PRESENTATION = RO) the red part is the same instance name (essentially different database services ). The other descriptor starts with a different name than the previous descriptor ....