«وَزِيَادَةُ رَاوِيهِمَا — أَيِ الصَّحِيحِ وَالْحَسَنِ — مَقْبُولَةٌ مَا لَمْ تَقَعْ مُنَافِيَةً»
An addition transmitted by a narrator of a sound or hasan report is accepted so long as it does not conflict with the other account.
Rawiyahima refers to narrators of both sahih and hasan reports. The inquiry therefore concerns reliable transmission, while taking account of the particular narrator's precision and the strength of competing reports.
An addition is incompatible where accepting it would require rejecting the alternative account in the same respect. A non-conflicting addition can stand alongside the shorter version; their treatment is different.