Add a second domain as an alias?

[SammyBar]

Hi all,

I'm configuring my zimbra installation to work as a secondary server on a split domain where Exchange 2007 is the primary autoritative server. This arrangement should last a few days-weeks in between I migrate all the accounts from the Exchange to Zimbra. I followed the instructions published here (Split domain with Exchange 2007 as Primary - Zimbra :: Wiki). Everithing works as expected.
The problem is we have two domains: mydomain.com.mx and mydomain.mx with users at the Exchange server using both domain for the same account. In the current configuration I installed Zimbra with the domain mydomain.com.mx and added the required alias zimbra.mydomain.com.mx required to route mail from Exchange connector to zimbra. Then I added one additional alias mydomain.mx to make zimbra accept users to use indistinctly both mydomain.mx and mydomain.com.mx. I made some test and it works for now. But I fear when I finish the migration and remove the split domain configuration it will not work.
Should I add mydomain.mx to Zimbra as a second domain instead as an alias?
Which is the reccomended configuration?

Thanks in advance
Sammy