In his autobiography, My Last Sigh (1983), Buñuel explains (pp. 46–47) the decision to use two actresses to play Conchita:
In 1977, in Madrid, when I was in despair after a tempestuous argument with an actress who'd brought the shooting of That Obscure Object of Desire to a halt, the producer, Serge Silberman, decided to abandon the film altogether. The considerable financial loss was depressing us both until one evening, when we were drowning our sorrows in a bar, I suddenly had the idea (after two dry martinis) of using two actresses in the same role, a tactic that had never been tried before. Although I made the suggestion as a joke, Silberman loved it, and the film was saved.
The actress who caused the 'tempestuous argument' was Maria Schneider. Bouquet and Molina stepped in after she was fired.