Do you need to have replies sent to more than one email address? While you can include the other address in the CC field and hope the recipients remember to choose Reply to all (or use rules to forward messages with the specific subject line to the other addresses), you can configure the message options so that the recipients can hit Reply and send the response to multiple addresses.
To do this:
When composing the message, click the Options button and enter the addresses in the Have replies sent to field on the Message options dialog.
Wasn't that easy? It's definitely much simpler than using rules to forward and you don't have to reply on people hitting Reply to All.