You can achieve this by creating domains. These are the steps to do so.
1) Add the external User to the user.xml file on the MOM.
2) Update the agent mapping of the current custom domain that includes the Specific Application Server to remove those from this existing domain.
3) Create a new custom domain, set the agent mapping to include the Specific Application Server only and grant permissions to the external User.
4) Finally,restart the Introscope EM, connect to the CA APM Workstation. Then, you should see the new custom domain that has the 2 servers only.
Note: An agent cannot belong to TWO Domains.