Maybe I'm making this too complex by going the virtual user route.
I only have a handful of users to worry about. The easier way may be to just add real users to Server-A and set up email accounts there for them.
dns on Server-A would look like this:
allparts.com and www.allparts.com
would have A-records that point to the IP for Server-B
mail.allparts.com would be added as an A-record and MX record on Server-A for mail
If I do that I would still have the full functionality of ISPConfig.
I don't know how this setup would handle users that need to send and receive emails on both domains. ie- email@example.com
I like the potential simplicity but, I'm still not sure how to do all of this properly.