Two or more nouns are connected by what punctuation if they are used as adjectives?
A noun functioning as an adjective to describe another noun is called an attributive noun (or a noun adjunct) . NO punctuation is used to connect the nouns.
Examples of attributive nouns:
the boy king
a child prodigy
a government agency
the morning sun
I believe that the question is referring to an appositive , a noun, a noun phrase, or a pronoun that renames the noun just before it.
An appositive has a comma before it and after it.
Examples of appositives:
Arthur, my cousin, gave me his old bicycle.
The winner, you, will take home the trophy.
That book, the blue one on the left, is due at the library.
The notice for the past due bill, mine, has been taken care of.
Mr. Moses, a neighbor, has a dog as big as a small horse.