How can one differentiate between fasting that is intended for its own sake and fasting that is not, in order to validate the intention of combining fasts?

The principle for combining acts of worship is based on intention: one of the acts should not be intended for its own sake, but rather included with another by intention. This applies to fasting and other acts of worship. What is intended for its own sake cannot have its acts of worship combined. However, if one of two acts of worship is not intended for its own sake, then it is permissible to combine it with another act of worship in a single intention, with the primary act subsuming the secondary. The decisive factor in this is that the intent of the Lawgiver is fulfilled by the occurrence of the action. This regulation is subject to the scholarly discretion of jurists and what they deem preponderant from textual sources and the fundamental principles of Islamic law.