The primary role of waw is simple conjunction; the primary role of fa is connected succession. Either may have a recognized contextual secondary use, but that shift must be justified.
Waw: “Zayd and Amr came” — both arrived, order unspecified.
Fa: “Zayd entered, then sat” — entering precedes sitting.
Figurative fa: “I owe a dirham, fa another dirham” — both sums count in the acknowledgement.
Begin with the particle’s ordinary function, then decide whether the circumstances establish a different legal or linguistic effect.