IIS OWA and multiple domains

We have a domain called mydomain.com which allows email access thru OWA. We added a second domain called mydomainna.gov and also want OWA access. What is the easiest way to accomplish this? I would like to use host headers in IIS, but since we require SSL, I do not think this is possible. I am trying to avoid adding a second IP address, if possible. Any thoughts/suggestions are greatly appreciated.