I think this is a DNS issue, nothing to do with Zimbra.
Suppose your server IP is 88.00.77.66.
Your DNS settings should look like this:
mail.myserver.com A 88.00.77.66
webmail.myfirstdomain.com A 88.00.77.66
webmail.myseconddomain.com A 88.00.77.66
If you want to login:
https://webmail.myfirstdomain.com/
Or:
https://webmail.myseconddomain.com/