Hi Ed. Like you I have never seen tank containers double stacked or more.
I have been in ports where they are in singles and not stacked like ordinary containers.
I have never seen them on box boats, but then never took any notice. Photographing cruise ships ‘is my thing’.
To travel by ship I would think they are stacked. What regulations etc. I do not know. They must get from A to B.
Sorry for not being much help.
David