The doubling technique definitely serves the purpose of both comparing and contrasting. Throughout the novel, he is trying to show the dual nature of man -- man can be a villain and a hero simultaneously. Basically, one person's revolution is another person's massacre. Clearly he is contrasting the major characters by developing so many foils - Carton and Darnay, and Lucie and Mdm Defarge are the two most obvious ones. By showing their extreme actions, reactions, and resolutions Dickens is able to pinpoint what causes a person of dual natures to choose one over the other. It boils down to the differences between love and hate.
Dickens uses the cities to be more of a comparing points. Similar people and similar circumstances are found in both. Again, though, love and hate separate the progressing of one country from the destruction of another.