[This Answer spills over to two posts...]
It all has to do with quotation marks. The voice that introduces the story--the Authorial Voice--never leaves the narrative, even while it would seem that Marlow is telling the narrative--as the Narrator--Marlow is in fact being quoted by the persona of the Authorial Voice. In this sense the two, the Authorial Voice and the Narrator, are one and the same.
However, to say exactly that Marlow plays a dual role as both the Narrator and the Authorial Voice is inaccurate unless we have reason to believe that Marlow is in fact telling his own tale separated from the event by great time and in disguise.
I say this because Marlow is never there in the storyline, which is that the Authorial Voice persona is telling a tale to unidentified listeners: Marlow is always just a memory. As far as I can see, there is no place in the narrative that the authorial voice persona equates itself with Marlow's identity. In fact, the last passages of Part 3 (quoted below) give just the opposite indication: The Authorial Voice and Marlow are entirely separate personas and entities. Therefore, the Authorial Voice can double for Narrator (as it is his memory and his retelling of Marlow's story), but Marlow--a memory--can never double for the Authorial Voice. although his narrative--his speech as Narrator--is told through the Authorial Voice.
Since Marlow is never, in fact, there with the Authorial Voice persona--he is only a memory--it would be far more accurate to say: The character of Marlow is cast in a dual persona. He is first the Authorial Voice persona's memory narrative and second the voice of the original narrative--as its Narrator--because the Authorial Voice persona's experience and Marlow's narrative are being quoted exactly as experienced and heard. In conjunction with this, it may also be said that the Authorial Voice persona is also the Narrator who is narrating to his unidentified listeners the tale of his experience with Marlow. The Heart of Darkness is a narrative inside a narrative so there are two Narrators, the Authorial Voice persona and Marlow, who is recalled as a memory.
There is an Authorial voice distinct from Marlow's, as illustrated by the following quotes:
Forthwith a change came over the waters, and the serenity became less brilliant but more profound. ...
"And this also," said Marlow suddenly, "has been one of the dark places of the earth."
He was the only man of us who still "followed the sea." ...
"...I wonder, if I had rendered Kurtz that justice which was his due? Hadn't he said he wanted only justice? But I couldn't. I could not tell her. It would have been too dark--too dark altogether. . . ."
Marlow ceased, and sat apart, indistinct and silent,... I raised my head.
[Second post to above Answer...]
Second: Authorial voice is defined as the persona who connects with the reader telling who did and said what, where, why, when, etc.
Third: Narratorial voice, or Narrator, is defined as the persona who tells the events of the story. Most often these are the same: authorial voice and narratorial voice are most often the same voice. A prime example of this in Jane Austen's works in which the authorial voice tells the narrative and makes a distinctive contribution.
Fourth: While authorial voice and narratorial voice are most often the same, they may also be two separate entities. As the quotes above show, the authorial voice that makes contact with the reader is not the same as Marlow's voice during the long narrative passages.
So, in summary: Even though there are a separate and distinct Authorial Voice and Narratorial Voice, (Narrator of Marlow's tale) and even though Marlow, the Narrator, is not the Authorial Voice, whilst Marlow is speaking, he fills the function of Authorial Voice because he is establishing contact with the reader even though simultaneously establishing contact with the listener on the boat to whom he is telling the story and who is now retelling it.
The fact that Marlow's speeches are always set off with double quotation marks ( "..." ) underscores that Marlow is telling the narrative to listeners on the boat one of whom is retelling the narrative to listeners. Nonetheless, during his narrative, Marlow may be considered to fill the function of Authorial Voice and Narrator. Again, The Heart of Darkness is actually a narrative within a narrative. In the inner narrative, Marlow is the Narrator and his story is communicated through the Authorial Voice's persona. In the outer narrative, the Authorial Voice persona is the Narrator telling a tale to his own listenters.