Dickens begins this book by famously comparing London and Paris. He chose to write about the French Revolution because he believed that conditions in London were becoming as bad as they were in France at the time. His purpose was to elucidate the causes of the revolution and the consequences of mistreating the poor masses.
[In] short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only. (Book 1, Ch 1, p. 4)
By having his characters travel between London and Paris, Dickens reinforces both the similarities and the differences. The book is full of paradoxes and comparisons. The title is a way of referencing and introducing these.
The novel gives the perpective of the characters' lives both in the city of London and in the city of Paris. Throughout the story, we see the characters' fates change depending on the city in which they are living or visiting before and during the French Revolution.