I suppose that it is easier (simpler) to modify 11g's LISTENER.ORA (i.e. adding 1523 to it, as described above) than to modify all TNSNAMES.ORA files, everywhere ...

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = dbs1)(PORT = 1521))
        (ADDRESS = (PROTOCOL = TCP)(HOST = dbs1)(PORT = 1523))
      )
    )
  )

You'll be a DBA soon

You might want to read the series of TNS related articles I wrote at edstevensdba.com

Exactly! The same situation here (apart from the fact that both databases run well). Thank you, I understand what you are saying.

I'll do that during the weekend - shut down 10g, set its listener to "manual", restart 10g, check whether I can connect to it using 11g's listener. Although, it's a mystery to me - how is it going to work OK all by itself, automagically? Do I have to change some settings, somewhere?

I'll let you know the outcome.

OK, I fixed that (DBSNMP and SYSMAN users have been EXPIRED; I altered them using the USER$.SPARE4 column value).

• connect to SQL*Plus as SYS
  

  alter system set local_listener = 'dbs1:1523';

• in Windows Services, STOP OracleOraDb10g_home1TNSListener and - in its properties - set STARTUP TYPE to "Disabled"
  
• add the following line into 11g's LISTENER.ORA file:
  
  LISTENER =
  (DESCRIPTION_LIST =
  (DESCRIPTION =
  (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
  (ADDRESS = (PROTOCOL = TCP)(HOST = dbs1)(PORT = 1521))
  (ADDRESS = (PROTOCOL = TCP)(HOST = dbs1)(PORT = 1523))
  )
  )
  
• in Windows Services, RESTART OracleOraDb11g_home1TNSListener
  
• check listener status (excerpt)
  

  D:\>lsnrctl status
  
  LSNRCTL for 32-bit Windows: Version 11.2.0.3.0 - Production on 21-AUG-2016 19:56:31
  
  STATUS of the LISTENER
  ------------------------
  Listener Parameter File   D:\oracle11\product\11.2.0\dbhome_1\network\admin\listener.ora
  Listening Endpoints Summary...
    (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\\.\pipe\EXTPROC1521ipc)))
    (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=dbs1)(PORT=1521)))
    (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=dbs1)(PORT=1523)))
  Services Summary...
  Service "ora11" has 1 instance(s).
    Instance "ora11", status READY, has 1 handler(s) for this service...
  Service "ora10" has 1 instance(s).
    Instance "ora10", status READY, has 1 handler(s) for this service...
  

• test connections to both databases

  SQL> connect scott/tiger@ora10
  Connected.
  SQL> connect scott/tiger@ora11
  Connected.
  SQL>

OK then (I'll be a DBA soon LOL!)

---

Meanwhile (in the past few minutes), I took the steps which were supposed to eliminate ora10 listener and make ora11 listener serve both databases. Everything went well, apart from the fact that I should have first modified LOCAL_LISTENER parameter and then stop ora10 listener itself because stopping the listener made impossible connecting to the database.

Hmmm, on a second thought, remembering what you guys used to say, perhaps it is irrelevant. When I'm on a server (I mean, connected directly to it (either by sitting behind its keyboard or using Remote Desktop)), I *think* I don't need listener at all.

So, the steps taken: