The book contrasts the two constructions:
Form IIIForm VI
شَاتَمَ زَيْدٌ عَمْرًاتَشَاتَمَ زَيْدٌ وَعَمْرٌو
Zayd: subjectZayd: subject
ʿAmr: accusative objectʿAmr: coordinated nominative
Both can express reciprocal insulting, but they encode the participants differently. Reciprocal meaning does not require both participants to appear as grammatical subjects in every morphological form.