Don't know if you're planning to name it off your current domain/what your naming convention is or if you even let them interact, but you can also setup a virtual hostname of webmail.example.com and configure mail.example.com (existing) to mail2.example.com (new).
When users log on to webmail.example.com, the load balancer directs the user to any one of the mail servers to verify the log on information. After successfully logging on, users are redirected to the actual server their mail is stored on. While they are logged on, all subsequent requests go directly to their server.
In order to configure load balancing for ZCS:
1. Each Zimbra servers must have a routeable address/name.
2. You must configure the virtual hostname on the administration console.
3. You must turn on the following localconfig setting on each mail server: zmlocalconfig -e zimbra_auth_always_send_refer=true
You also have to decide if you want to use IMAP and/or HTTP nginx proxy. But if they're totally separate you might not care
/docs/ne/latest/multi_server_install/toc.html