Enterprise Manager Configuration Assistant - Oracle 10g/11g Data and Database Management Utilities - page 342

You can check the EM Console configuration files to find out if the selected port
has already been configured for another EM console. The configuration files are
located at <ORACLE_HOME>/<hostname>_<OracleSID>/sysman/config/emoms.
properties , there you will find an entry named oracle.sysman.emSDK.svlt.
ConsoleServerPort , which declares the port used by the console.
The emoms.properties file cannot be directly modified by the user,
the EMCA command must be used to set the EM http Port.
If the instance is not running then the EMCA will return an error and won't perform
the configuration task.
Once you are ready, just issue the command below with the DBCONTROL_HTTP_PORT
parameter set.
emca -reconfig ports -DBCONTROL_HTTP_PORT 1158
In the following image, the command has been issued ( A ), all actions were logged at
the given path ( B ). If something doesn't work as expected, this is the first place you
should go to get feedback.
In this particular scenario a duplicate port was selected. The EMCA notices the
problem and shows the user a warning ( C ) alerting the user to the fact that the port
is already in use. The operation will finish, as this is not a fatal error, but if ignored it
will allow only a single EM Console to work.