Since some of those mentioned in other answers are among the living, let me also mention Victor Kac and his teacher Ernest Vinberg. They published a joint paper Spinors of 13-dimensional space in Advances in Mathematics 30 (1978) under the rather transparent pseudonyms V. Gatti and E. Viniberghi. As I recall, Victor said that this came about because he had applied for an exit visa from the USSR and was therefore not allowed to publish anything in the interim.