You can have more than one domain on the Domino Side. The BES is an add on for Domino, which means it follows the entries on the NAB and domino and not vice versa.
Why would someone have 2 email addresses on the corp side / Desktop?
Why not go the route of a BES and BIS address like most people have set up ie Corp Email and Gmail account / yourowndomain.com
1. You dont have the field in names.nsf to support 2 internet address unless you do custom.
2. In my opinion, you would need another BES server or unless you create this address as a completely new person doc and register on the current bes with a diff handheld but this is too much for one user to carry 2 devices.
At the end of the day, having 2 email addresses can get messy. I would go the bes and bis route versus complicating the environment as such especially if its for one executive of the firm / head hancho vs majority of the user's.
Technically, you could set up a separate person doc for the second account and have it autoforwarded thru the forwarding address field, or even set up a rule in a database to forward it - but that's just fugly, and I certainly wouldn't recommend either one - it's an administrative nightmare waiting to happen.