فَإِذَا زَالَتِ الشَّمْسُ مِنْ يَوْمِ عَرَفَةَ صَلَّى الْإِمَامُ بِالنَّاسِ الظُّهْرَ وَالْعَصْرَ.
Translation: When the sun passes its zenith on the Day of Arafah, the imam leads the people in Dhuhr and Asr.
Zawal is the sun's passage beyond its zenith. Al-Quduri describes the imam leading both Dhuhr and Asr during the Arafat rites.
The subsequent sentence specifies that this combined congregational performance occurs within Dhuhr time. It is a special Hajj context, not an unrestricted rule for ordinary days.