If I understand the question correctly, you want to be able to connect to the 10g database using your 10g client and to the 9i database using your 9i client --- even though you can currently connect to both databases using your 10g client. Mine is not to question, "Why?" The answer is two listeners, one instantiated from the 10g ORACLE_HOME and the other from the 9i ORACLE_HOME. Don't put the 10g database in the 9i listener.ora and don't put the 9i database in the 10g listener.ora. It's perfectly possible to run two listeners simultaniously from different ORACLE_HOMEs as long as you don't try to contact the same database(s) from each.