Anarres and Urras – Ursula K. Le Guin’s The Dispossessed
Ursula K. Le Guin’s The Dispossessed is an absolute classic of sci-fi, and the twin planet system of Annares and Urras is brilliant. Urras is a mostly capitalistic society full of different civilizations, and the other is an anarchistic society that split off from the other planet. There is a lot of tension between the two worlds, as each considers the other their moon, rather than acknowledging them as separate planets. It’s seriously awesome-sauce, guys!