From two mothers, the two halves of his body come out separately. And in consequence of those two halves being united by that Rakshasi, he was called Jarasandha.
द्वाभ्यां जातो हि मातृभ्यामर्धदेहः पृथक् पृथक्। जरया संधितो यस्माजरासंधस्ततोऽभवत्॥