To connect to a different Oracle Database (if you already connect to one) all you need to do is go into you Oracle_Home directory. Then go to network\admin\ in this directory you will see a file called tnsnames.ora you need to edit this. Copy the record that is in there. Edit the second one by changing the machine name and sid. If you have an Oracle DBA on site they should be able to assist in doing this.

*SID*.WORLD =
    ( DESCRIPTION =
       (ADDRESS_LIST =
           (ADDRESS = (PROTOCOL = TCP)(HOST = *HOSTNAME*)(PORT = 1521)
+)
        )
      (CONNECT_DATA =
        (SERVICE_NAME =  *SID*.world)
      )
  )
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