Flying with TL on (other than for OEI Training) in case you accidentally overtorque the gearbox seems very little difference from deliberately flying with only one engine in case you accidentally overtorque the gearbox. It doesnt compute with me.
Its not in the manual, its not taught on the TR course, because that is not how it is intended to be used. The manual doesnt say "dont turn both generators off in flight to save wear and tear" either. That doesnt mean to say you should do it!