To have one role played by two actresses prompts one to speculate that the director intended to have different aspects of her personality shown by the one or the other. When asked about this, however, Fernando Rey told me personally that was not the case. They merely had two actresses they liked and decided to use them both because they couldn't pick obe over the other. Considering Luis Bunel's surrealistic inclinations throughout his career -- and throughout this film -- there was no question Rey was correct. After all, he was there and anyone who knows women knows what you get one day may not be what's there the next.
What inclination did Luis Bunel have?
Surrealistic