Yes you can setup 2 location documents that would switch to the correct id file.   Other options would be to cross certify the one id to access the other server but that might be an issue if the user needs the ability to send email using the other email address.

The location document sounds easy.  So if the user wants to check mail from domain1, he would have to log onto the Domino server with the user id file from domain1.  Is that correct or is all of this taken care of automatically by having the two different location documents?

On the location doc you can specify what id file you should switch to for that location

Thanks very much, definitely appears very easy.  I appreciate all the input.