There are indeed two narrators in Heart of Darkness. First of all, there is the narrator of the frame story which introduces us to Marlow. This is told by a sailor aboard Marlow's ship, the Nellie, as it lies moored on the River Thames. Then of course we have Marlow himself, who proceeds to tell the other passengers the astonishing tale of his adventures in Africa.
The main reason why Conrad uses two narrators in this way is to highlight one of the book's most important themes: the contrast between savagery and civilization. The Nellie is a humble merchant ship; it is certainly not the kind of vessel that one would expect to see making its way up the Congo into the eponymous heart of darkness. It is safely moored on the Thames in the heart of Southern England and is as far removed as it's possible to be from the harsh reality of life in the Empire.
This serves to make Marlow's extraordinary story all the more exotic and fantastical. But it also allows the unnamed frame story narrator and the other passengers to maintain a distance between themselves and what Marlow's story reveals about the sordid realities of life in the Empire.