A tale of two servers

Seeking suggestions on the following scenario: Two OS X Server installations, each in a different city on a CDSL line hosted by different ISPs. What would be the best way to handle mail? 1. Separate domain names and then use normal server setup for each one? 2. Is there a way to have one server receive all mail and forward on to the second one mail for users on that server? Example: domain = mydomain.com. In "mydomain.com" I have "server1.mydomain.com" and "server2.mydomain.com". Each server has unique user accounts. How can I have mail for the users sorted for delivery to the different servers? Inbound mail would be addressed to "user@mydomain.com". What process would be used to determine if the mail goes to server1 or server2? This may seem to be an elementary question, but I am looking for the most efficient / elegant way to do this and would like some suggestions. Thanks