Once the agent is installed for a specific domain, the agent binaries cannot be used on that same installation for a different WebLogic Server/Portal 10 domain. If you attempt to use previously installed agent binaries on the same installation, but on a different domain, the installation fails.
J2EE agents associate a specific set of agent binaries with a particular domain for WebLogic Server/Portal 10. If you want to install a J2EE agent on a different domain, unzip a new set of bits and copy them to a separate location before running the agentadmin --install command for the second domain.