The affix ṇvi comes after the verb bhaj 'to share' when in composition with a word ending in a case-affix whether it be an upasarga or not.,
The words 'upasarga' and 'supi' are understood in this sutra. As अर्द्धं भजते = अर्द्धभाज् nom. sing. अर्द्धभाक् 'sharing a half'. So also when in composition with a preposition; as प्रभाक् 'division'.,