This is of course a very important question, as clearly London and Paris are the two cities that the title of this work refers to. However, what is more interesting is the way in which the two cities draw our attention to the role of doubles in this story and how important they are. Let us remember the way in which the opening of the novel, "It was the best of times, it was the worst of times..." supports this approach. The way that the action of the tale is split between London and Paris also reinforces the way that various themes are presented in the novel. Consider the way that this allows Dickens to juxtapose characters such as Lucie and Madame Defarge, who are clearly doubles for each other. This juxtaposition allows Dickens to point towards various thematic ideas that are central to the novel. Consider the way that Lucie's love for her father enables him to be resurrected from his mental instability, whereas Madame Defarge's unyielding commitment to vengeance only serves to create and endless cycle of violence, revenge and oppression.
Another theme that the technique of doubling helps establish can be seen in the character of Sydney Carton and the way that doubling helps establish differences but also can surprise us by pointing out similarities. Carton, in spite of his appearance, seems to be a foil for Darnay in terms of his character. Yet, as the end of the novel shows clearly, Carton has the ability to move from being a good-for-nothing dissolute character to evincing even more heroic qualities than Darnay himself through his self-sacrifice. The doubling that is indicated by the two cities of the title thus helps consolidate one of the central and most powerful themes of the novel: every human has the potential to distinguish himself through a radical act of goodness, no matter how unpromising that individual is.
We can therefore see through an examination of the important motif of doubling, which is of course presented to us first of all in the title, that Paris and London are used to enhance the presentation of key themes in this brilliant novel.