The twin planets consist of Metallokis I and Metallokis II.
These two planets were created thanks to a collaboration between scientists and astronauts; they created them as part of a social experiment titled Substantia Naturae (Natural Substance). The Twin Planets were one of the major reasons behind the Second Relocation; it would be a shame to have created something as amazing as a planet and have no one to inhabit it.
The term Twin Planets tends to bring bad memories to all who hears it.
The Twin Planets were the beginning of the end in a sense; without them, the Great War might never have come to pass.