Andrew,
Here is the KB that tells you how to add a domain :
It is that way for the 1st, 2nd, 3rd, etc..
I assume you have a licensed copy of SmarterMail and not the free version, as the free one only allows one domain.
If you point both domains to the same IP address, MX records, it should delivery mail to the 1st domain when sending to @xyz, and when sending to @abc it should delivery mail to the 2nd domain. If you were to configure MX records pointing to that same IP for a 3rd domain, when sending to that domain lets call it @qrs, it should bounce saying relay denied or user unknown, as that domain isn't setup on your mail server.
There should be no way @xyz and @abc accept mail for each other, unless a domain alias was added to them, it would accept any @domain.com if the alias was setup as it.
There must be some type of configuration issue that is making this do this.
Cheers,
-dave