You will need to purchase a UCC SSL Certificate and install that. A UCC SSL Certificate pricing is based on how many domains you intend to use the certificate for.
You would need 2 for your example above, and since Zimbra only supports installation of 1 SSL Cert, you will need a UCC SSL Cert to secure more than 1 hostname.
I have successfully installed UCC SSL Certs from GoDaddy, and am running one currently.
Hope that helps.
1+ Year on Zimbra
Release 6.0.6_GA_2330.RHEL5_64_20100505193959 RHEL5_64 NETWORK edition.
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