In ADA II-8 (418-22) the morning  after the nightclub scene Van  finds himself abed with both sisters.  Nabokov writes:

What we have now is not so much a Casanovanic situation (that double-wencher had a definitely monochromatic pencil — in keeping with the memoirs of his dingy era) as a much earlier canvas, of the Venetian (sensu largo) school, reproduced (in ‘Forbidden Masterpieces’) expertly enough to stand the scrutiny of a borders vue d’oiseau (418).


In his notes to Kirichenko Russia translation of ADA, Nikolai Mel'nikov identifies the Casanova allusion as Casanova account of his dallliance with the two


Giacomo Casanova

The memoirs of Jacques Casanova de Seingalt


An Unlucky Night I Fall in Love with the Two Sisters, and Forget Angela—A Ball at My House—Juliette’s Humiliation—My Return to Pasian—Lucie’s Misfortune—A Propitious Storm