Trigger notifications when Requester is an agent

Can someone help me figure out how to set up a trigger to send an email notification to the requester when the requester is an agent; they mention another agent in their ticket; and that agent responds via email, which creates a private reply? We don't want end-users to see private replies, so I understand the default functionality, but when the requester is an agent who can see the private replies anyways, it would be nice if they were notified of the update.