Follow-up #1 - So I could go into the manager.properties file and change it to match however many clients I currently have deployed, restart the server, then go to my Agent listing, Select all, Actions, Change Endpoint Server, enter the Primary server and port, hit the +, enter the secondary server and port, and OK and save back out. This should add the secondary server to all agents. I did a test walkthrough of this and it seems that it will work just fine, with the caveat that it will only work on connected agents. If the agent was Shut Down, or just plain Down, as in Agent Lost Connection, you get a message that the task can't be done.
Follow-up #2. OK, so we could go into Server #1's config and tell it to handle no more than 3000 agents, and then any agent that tried to connect to it after # 3000 would go to Server #2, right? That way we could ensure that Server 1 never got overloaded. Sounds good to me.
Thanks very much for your help!