A word joined with (or governed by) the word antarā or antareṇa takes the second case-affix.,
The anuvritti of dvitiya is understand here and not that of tritiya. Both these words antara and antarena are Nipatas. They govern the accusative. This debars the genitive case. The word antara means 'between' while antarena means 'besides that,' 'without,' 'exception,' 'with reference to' 'regarding.' As, अन्तरेण पुरुषकारं न किंचिल्लभ्यते nothing can be gained without exertion. अन्तरा or अन्तरेण त्वां च मां च कमंडलुः 'the bowl is between thee and me.' कोऽन्यस्त्वां मां तरेण शक्तः प्रतिकर्तुम् 'who else but thee is able to retaliate.'
Why do we say 'when joined with ?' Observe अन्तरा तक्षशिलां च पाटलिपुत्रं स्रुघ्नस्य प्राकारः.,