«وَإِنْ رَوَى كُلٌّ مِنْهُمَا أَيِ الْقَرِينَيْنِ عَنِ الْآخَرِ فَهُوَ الْمُدَبَّجُ»
“If each of the two peers narrates from the other, that is mudabbaj.”
Two conditions combine: first, the transmitters are peers; second, each actually transmits from the other. Merely meeting, or transmission in only one direction, does not suffice.
For example, Aḥmad narrates from Khālid in one isnād, and Khālid narrates from Aḥmad in another, while they are peers. Their relationship is mudabbaj. It is not required that both report exactly the same hadith; the criterion is reciprocal transmission.