I'm not sure that I know of any multi/seaplanes for rent or training. You might try Randy Hanna at Seaplane academy in Nanaimo, BC +1-250-616-5885 as he used to run a twin of some sort on floats.
Randy had both a Beech 18 and a Aztec Nomad on floats, however he sold that company and has just received his operating certificate for sea plane training both single engine and multi engine.
Single engine training is offered on a Cessna 180 and also training on a Husky Amphibian is available.
The multi engine training is available on a Beech 18.
Personally it is my opinion that spending money on flying something as expensive as a Beech 18 is only justifiable if you have a job prospect in mind that requires being able to fly a multi engine sea plane.
And a lot of how to operate a multi engine sea plane can be learned without actually getting in the air.
So be wary of spending a lot of money on multi engine training unless you are sure you need it.
Randy's phone number is:::
250-884-8184