Mudabbaj is commonly established by comparing two distinct isnāds. In one, Zayd receives from ʿAmr; in the other, ʿAmr receives from Zayd. The two chains disclose reciprocal transmission.
Isnād 1: Narrator A ← Zayd ← ʿAmr ← his teacher.
Isnād 2: Narrator B ← ʿAmr ← Zayd ← another teacher.
Here the arrow means that the narrator on its left reports from the person on its right.
If the chains are established and Zayd and ʿAmr are peers, the classification is mudabbaj. The existence of one chain does not justify inventing its reverse.
Al-Dāraquṭnī produced a dedicated work on mudabbaj. This reflects the importance of comparing actual transmission routes rather than inferring reciprocity from a single direction of narration.