On the daf / in context: Because the entire permission rests on genuine desire (the yetzer-hara concession), the Torah requires that the soldier’s desire be focused on one woman alone. Desire for two at once shows that the craving is not the kind the Torah accommodates.
Type of Learning
Mi’ut (exclusion) from the singular pronoun “bah.”
What Is Learned
The captive-woman permission applies only when the soldier desires one specific woman, not two at once.