Before you seek out what is probably very rare training, determine if you need it. I have a multi engine seaplane rating in Canada, though I have never flown one here, only singles. By regulatory acceptance in Canada, if you have a land multi engined rating, and a seaplane rating, the combination of the two allows you to fly multi engined sea. Check if it is similar for your authority. For the purpose of of multi engined flight disipline, the landing gear is hardly relevant.