We've had various stakeholders that want to co-manage their tickets together, or a supervisor just wants the updates to read in their email box without replying. We worked around that by advising them to set up an alias on their email system to forward to both of their accounts, and to create an identity also so if they do reply, it reflects a single email address. We have the email fetcher/ticket creator disabled and implement registered accounts only which works for our business model, and also as a rather positive consequence, it eliminated spammy fake tickets. And rather than including the response in the email body, we force them to view the response in the ticket and reply in the ticket. ie: "You have an update, click here/log in." This also helps with keeping any private or sensitive information out of email and behind an SSL cert.