I have not flown a Twinbee, though the though of an aircraft which could be flying at 3800 pounds on 180HP un nerves me. Knowing that that aircraft has the aerodynamics of a flying boat is more worrisome. I have lots of light flying boat time, and they are always an aerodynamic compromise. I trust that the instructor was very experienced with single engine ops in this aircraft, but it must require a lot of skill! I think of flying a loaded Cessna 207 at less than two thirds power, and that would be symmetrical! I guess that it would be similar to a Twin Comanche on one engine, other than a Twin Comanche is pretty aerodynamic, and optimized for single engined flying - and still has a reputation!
Interestingly, in Canada I have a multi engine seaplane rating, though I never trained on a multi engine seaplane. The combination of a multi landplane, and seaplane rating, combine to cover multi seaplanes without additional training. That's probably a good thing, because aside from the Twinbee, and Aztec on floats, other multi engined seaplanes are much more capable on one engine. I think that training single engine flying in a low power twin amphibian is a risk without commensurate benefit.