As I understand it, kr̥ṣṇa-dvaipāyana is a compound of two words, kr̥ṣṇa and dvaipāyana. He is called dvaipāyana, since he was born on a dvīpa "island" in the Ganges from the union of the sage Parāśara and Matsyagandhā. The word dvaipāyana has the Vr̥ddhi of the first syllable as one sees in many Taddhita formations. It then gets connected to the word kr̥ṣṇa as two adjectives forming a compound. That is why we do not see the Vr̥ddhi in the word kr̥ṣṇa.

     Best wishes,

Madhav