In fasting, the assigned day functions as a measure (miʿyār): a single day cannot serve as the complete performance of several independent fasts at once. This differs from the enveloping function of prayer time.
The discussion of Ramadan distinguishes the existence of an intention from specifying a particular fast. Intention matters because merely abstaining from food does not by itself establish an act of worship. The conditions for specifying the fast differ across cases, including those of illness or travel.
Faulty analogy: Inferring that multiple obligatory fasts fit in one Ramadan day because more than one prayer can occur within a prayer period confuses two distinct temporal structures.