The following are sample entries of tnsnames.ora file.

here there are two databases, both named high9i. one is remote and one is local

configure the connect string as different.
eg:
for remote database= high9i
for localdatabase  = local

HIGH9I =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.27)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = high9i)
    )
  )

LOCAL =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = high9i)
    )
  )

SQL> connect scott/tiger@high9i
Connected.
SQL> connect scott/tiger@local
Connected.
SQL>