The divided city is as central to the postmodern urban imaginary as it is to the contemporary urban praxis. In addition to cities segregated along economic and racial lines, there are politically and ethnically separated cities, and there are also ‘virtual cities’, in which a core of highly mobile and technologically savvy population inhabits a different spatial reality from the surrounding struggling community. What all these divided cities have in common is the phenomenon of ‘lost spaces’, in which intermediary zones become a no-man’s-land in terms of the organising principle that structures the rest or the city. The lost space is necessary to separate the two entities and thus to ensure their purity. But it reveals the price of this separation by becoming a zone of lawlessness, impurity and danger. It is not a utopia of freedom but rather a black hole of social space. Urban science fiction and fantasy explore the divided city by translating topography into topology: that is, by treating different places as different spaces, each with its own set of physical laws. By literalising the metaphor of the “two cities in one”, such novels as Neil Gaiman’s Neverwhere (1996), China Miéville’s The City and the City (2009), and Tim Lebbon’s Echo City (2010), bring out the inherent paradox of the divided urban space: its attempt to achieve homogeneity and safety necessarily breeds liminal zones of lawlessness and violence. This chapter will explore the double-layered urban chronotope, focusing on Miéville’s The City and the City and Tim Lebbon’s Echo City. The urban division in these novels is re-imagined as a split within the fictional space itself. This split undermines our basic intuitions of spatial perception, so that the chronotope becomes paradoxical and self-contradictory and ultimately impossible. The poetics of impossible urban space in sf and fantasy as a reflection of the postmodern urban politics will be explored, It will be argued that the impossible dual structure of many fantastic urban chronotopes reacts to the problematic of divided cities as both increasingly central to the global economy and yet, in the long run, unlivable and unsustainable.