Exchange server send mail for 2 domains?

Let's start here: My customer has hosted mail and website using a domain I own "mydomain.COM".  My customer uses Outlook and Exchange, and they have a stupid setup.  Rather than purchasing a receive connector, they have 2 accounts in Outlook, 1 is their hosted email, the 2nd is their exchange server.  The storage folder for Outlook is the exchange server, so essentially, Outlook checks the hosted mail account, downloads the mail, and stores it on the exchange server.

I am setting up a new SBS2011 server for them.  To make OWA and Android phones work without prompting for a certificate, I decided it would be a good idea to buy a 2nd domain "mydomain.CO" and then buy an SSL certificate for my server as well so that it verifies OK.

I thought it would be cool if they could send email for "mydomain.COM" directly from the exchange server, rather than having to send it to the hosted server, then have the hosted server send it out.  

At this point, I want to be able to SEND (SMTP) from either domain "mydomain.COM" which is hosted elsewhere, or "mydomain.CO" which is hosted locally in my exchange.  How do I set this up so I can send from both domains from my exchange server and it be configured "correctly".