Can the novel be seen as an allegory? Is Sydney Carton's journey, to a certain extent, modelled after the "Pilgrim's Progress''. In that case, conversely, how does Charles Darnay progress?
If we're making comparisons, I can certainly see some similarities. I would argue that Charles Darnay is in fact the character Christian, then Lucy Manette would most definitely figure as Evangelist who helps guide him to the "shining light," allegorically speaking.
In some ways, I could see Sydney Carton as being more of the 'Faithful' character whom Christian meets in the Valley of the Shadow of Death (France), only because the similarity that both Faithful and Sydney are executed.
I really like the possibilities of this allegorical comparison; there are definitely many interesting similarities to draw upon and discuss.