The action in VN's novel Ada (1969) takes place on Demonia, Earth's twin planet also known as Antiterra. After the family dinner in "Ardis the Second" Ada mentions spies from Terra:
He kissed her half-closed lips, gently and ‘morally’ as they defined moments of depth to distinguish them from the despair of passion.
‘Anyway,’ he said, ‘it’s fun to be two secret agents in an alien country. Marina has gone upstairs. Your hair is wet.’
‘Spies from Terra? You believe, you believe in the existence of Terra? Oh, you do! You accept it. I know you!’