دَخَلَ الْمُحَقِّقُ يَحْمِلُ نُسْخَتَيْنِ.
The researcher entered carrying two copies.
Yaḥmilu nuskhatayni is an affirmative imperfect verbal clause describing the state accompanying entry. Its implicit subject refers to al-muḥaqqiq. In this construction the circumstantial verb clause suitably appears without the circumstantial wāw; adding wa-yaḥmilu would require a different contextual or syntactic explanation.
By contrast, dakhala al-muḥaqqiqu wa-huwa yaḥmilu nuskhatayni uses a nominal circumstantial clause with wāw and a pronoun. The comparison makes clear why the rule for an affirmative imperfect must not be applied indiscriminately to every clause containing an imperfect verb.