Dope,
In order to benefit from the cross over between SEP and TMG, both ratings have to be valid. i.e. you have in your licence a Certificate of Revalidation that is in date. If it is in date you can revalidate by experience or test on either device. If you do that by test the examiner can sign both ratings, he does not need to be qualified on both types, it is a revalidation and both ratings remain current throughout.
If you let one expire, and you are revalidating them on the same criteria, then both must have expired. The ANO says:
(c) Expired Ratings
(ii) If a single-pilot single-engine class rating has expired, the applicant shall
complete the skill test in accordance with the requirements specified at
Appendix 3 to paragraph 1.240 of Section 1 of JAR–FCL 1.
So you now have to renew each rating on each class of aeroplane. Then you can keep them both current using hours on either. The moral of the story is that if you have both ratings, keep them valid!
If the SEP had still been valid, and you did the TMG test to renew that rating, then you could have revalidated the SEP on the basis of that test, but again the examiner does not need to be dual rated.
The examiner can only test on classes of aeroplane he is qualified to fly, he can sign administrative revalidations on other classes specified in his Authorisation.