I think you need to consider pseudonyms and groups. This is where it gets interesting...
Take a user called "russ" with password "hello". And to keep it simple, assume you have domain1, domain2. Create an account for russ@domain1 in your mail client, using account=russ and Passw=hello. All mail sent to russ@domain1 AND russ@domain2 will land in this account. So far, you can only send mail under the identity russ@domain1. If you wish to send mail under the identity russ@domain2, create another account for russ@domain2.
Next, russ want to have another email address for domain1, say sale@domain1. Go into pseudonyms, and create an entry for "sale" and associated it with user "russ". All mail addressed to sale@domain1 will go to user russ. You can also create an account in your mail client for sale@domain1 for user "russ" (that is username=russ + password=hello), now russ can send mail under the identity sale@domain1.
Next you wish to set an email account called info@domain2, but you want mails to be distributed between you (russ) and your partner (paul). First of all, paul has to be a user. Next, create a group called "partners", members are russ and paul. finally, create a pseudonym "info" and associated it with the group called "partners". Now all incoming mail addressed to info@domain2 will go to user russ AND user paul - assuming paul has an account under his name in his mail client..).
Russ and paul can also create an account in their mail client under info@domain2 using their respective username and password, and send mail under the identity info@domain2
Hope it helps, sorry for longuish post.
chris
All this will work subject to mx records for domain1 and domain2 being set.