Today our development team needed a new report server instance in middle tier, in process of doing so they came up with following observation and steps:

Assume %OMH% is Oracle Middle Tier Home

To create new report server instance:
Open command prompt:
C:\>rwserver server=%newreportserver_name% start

Shut down the opmn:
C:\>%OMH%\opmn\bin\opmnctl stopall

Add new server target to opmn.xlm
C:\>%OMH%\bin\addNewServerTarget.bat %newreportserver_name%

Open command prompt again:

Update the config with new settings
C:\>%OMH%\dcm\bin\dcmctl.bat updateconfig -ct opmn -v -d
C:\>%OMH%\ dcm\bin\dcmctl.bat resyncinstance -v -d

Start the opmn
C:\>%OMH%\opmn\bin\opmnctl startall

Things to keep in mind:

Do NOT register the inprocess (aka in-process) server (ie. rep_) as it is already registered with the Enterprise Manager Console website and OPMN. Doing so will cause problems. The inprocess server is not intended to be viewed and/or stopped/started from command-line or opmnctl.

You must start or stop a Reports Server registered with Oracle Enterprise Manager 10g only through Oracle Enterprise Manager 10g/OPMN. OPMN automatically restarts Reports Server if it stops responding for some reason. On Windows, OPMN itself is run as a Windows service.
Beginning with Oracle Reports 10g Release 2 (10.1.2), running Reports Server as a Windows service is no longer supported (rwserver -install server_name). As a result, the related command line keywords INSTALL and UNINSTALL are also obsolete. If you start or stop a Reports Server that is managed by OPMN running as a Windows service or through the command line, you may face the following issues:
• The Reports Server’s status will not be reflected accurately in Oracle Enterprise Manager 10g.
• Oracle Enterprise Manager 10g may display errors when starting or stopping the Reports Server.