London plays a huge role in Bookhounds of London, obviously. But a feature of Lovecraftian cities is that they come in pairs: Ib and Sarnath. Irem and The Nameless City. Carcassonne and Carcosa. Samarcand and Sarkomand. Arkham and Salem. New York and Rl'yeh, even. Often one is entirely Lovecraft and the other is "real" yet also mythical or fantastical, this giving the Lovecraft invention the verisimilitude of association (there's also alien cities which tend to have their pair beneath them, in a trope which is far too freudian/marxist for my purpose).
So, my question is, what or where is London's twin?