n the self service portal a requester can make a request. The technician answers the request. After that the requester is able to reply to that message in/from the self service portal. When the requester hits the reply button the template of the email for the technician appears. (Organisation Details -> Notification Rules -> email templates: Reply request template.)

From the SSP perspective that is strange. The technician template contains text and lay-out that addresses the requester. It starts with "dear $RequesterName" for example. (See example below.)
There is no specific template for a requester who answers a technician message? Or any other way to get a correct template used by the requester and that addresses the technician?
Regards,
Olivier