Thank you for your reply Whopity.
Since a TRI candidate gets credit for the 25 hours of teaching and learning from the FI course, and both courses require the same assessment of competence in accordance with FCL.935, I am surprised that by undertaking a TRI course a current FI holder cannot revalidate both certificates with one event.
If I understand you correctly, a holder of several instructor ratings such as FI and TRI needs a separate refresher seminar for each rating. Once again the FARs offer a much more practical solution by allowing a CFI to revalidate all his ratings with a single flight instructor refresher clinic.