The Ark Class ship formed part of the three-fold survival plan of the wealthy and powerful of the Petroleum Age. The other two legs of the plan were the arcologies, and space habitats.
When night falls, we find peace in the knowledge that daytime will return soon, and vice versa. But what if the world was split: daytime and night are two different, coexistent worlds, each with it's own laws and rules. At dawn and sunset, our soul switches to inhabit our other being, our 'twin'.