What is the symbolic meaning of Roderick Usher and Madeline Usher and their interaction at the end of the story? What does Poe argue for?

It looks like you are asking about the symbolic meaning behind Usher and the Lady Madeline's interaction at the end of the story.
First, let us recount the last section of the story. Here, Madeline reveals herself to Usher (and the narrator). Apparently, she was buried alive, and Usher has been hearing sounds from her tomb for days. Fearing the implications of what he has been hearing, Usher neglects to share his concerns with the narrator. Meanwhile, a powerful gust of wind throws the doors open, and Madeline reveals herself.
Her white robe is bloodstained, testifying to "some bitter struggle upon every portion of her emaciated frame." Apparently, Madeline used every last shred of her strength to free herself from her tomb. Upon seeing Usher, she falls upon him, and in a last terrifying act of violence, she drains the life out of her brother.
It seems to me that Poe is making the argument for dualism in human nature. In the nineteenth century, many philosophical experts maintained that double consciousness was a human reality. They argued that the split between our private and public selves often resulted in neurosis or psychosis. Now, neurosis involves extreme anxiety, but psychosis goes a step further. An individual who is psychotic has lost touch with all reality.
This idea of dualism or double consciousness can be seen in other nineteenth-century works. Poe's "The Fall of the House of Usher" was published in 1839, while Robert Louis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde was written in 1886. In both stories, a double consciousness exposes latent psychosis in the male psyche. Certainly, the Victorian era strictly defined what a man should be and how he should act. Men like Usher, who were obsessive, insecure, and fearful, were said to suffer from a double or alternating consciousness. In such a state, a man's dark side openly fought for dominance.
It is, however, this dark side that the man must hide from society. In Poe's story, the final scene involving Usher and Madeline highlights the Victorian obsession with the duality of human nature. Poe's story ends similarly to Stevenson's. Just as Hyde dominates Jekyll, Usher is dominated by Madeline, his twin self. In a stunning twist, Madeline (the female representation of Usher) displays obvious masculine power in bringing down her brother. Poe's delineation of Usher and Madeline's last moments suggest that the psychotic, alternative self brings only self-annihilation.
https://www.bl.uk/romantics-and-victorians/articles/duality-in-robert-louis-stevensons-strange-case-of-dr-jekyll-and-mr-hyde