أَفْعَلَانِ is the masculine dual form, referring to two masculine entities possessing the quality to a greater degree. In the source's nominative paradigm, the dual ending is -āni.
Word / formMeaning / explanation
أَكْبَرُ ← أَكْبَرَانِtwo greater/bigger males.
أَصْغَرُ ← أَصْغَرَانِtwo smaller males.
أَعْلَمُ ← أَعْلَمَانِtwo more knowledgeable males.
These are nominative paradigm forms; the dual ending can change under other syntactic cases.