Why there are two narrators in heart of darkness?

There are two narrators cause it's written in the modernist genre, which uses indirect discourse to shift the narration from third to first person perspective. The other technique being used by Joseph Conrad is Free style indirect libre, which allows the reader to know what the character is thinking by the removal of tag lines, like "he said, Mr. conrad thought, or other way characters normally quoted so the reader knows they're talking. I tried my best

-J