Delegated Folders:
When you 'Delegate' a folder, you can choose a 'From Address' from your account when sharing the folder. The user will be able to read, comment and further respond or reply back to the emails, on your behalf. The Delegation, is allowing the user to reply to the emails in the folder on your behalf, with a chosen email address.
It sounds as if
ONE@domain.com can share their folder with
TWO@domain.com. When TWO sees a new email in the shared folder, they should be able to reply from the ONE address. I've set this up a couple times and I can never send from any other than TWO's accounts or aliases. Is there something I'm missing?