I'm by far NO expert on this, and only going on things that I've picked up.
I always understood it to be that the HIN needs to be on the transom in clear view, and then somewhere else on the boat in a more inconspicuous place. Not sure what year this was put in place, but I'm guessing in the mid 80's.
State of NY placed the interior one on my boat on the inside of a bulkhead - under the upholstery.

OH, and the two HIN's need to match