إِنَّ زَيْدًا ذَاهِبٌ وَعَمْرٌو
Certainly Zayd is departing, and ʿAmr too.
Inna governs its noun in the accusative: «زَيْدًا»; an ordinary independent subject is nominative. For «عَمْرٌو», one permissible analysis takes it as a new nominative subject with an omitted «ذَاهِبٌ»: إِنَّ زَيْدًا ذَاهِبٌ، وَعَمْرٌو ذَاهِبٌ.
An alternative attaches the nominative conjunct to the underlying syntactic position of inna's noun. Whether that is permitted before inna's predicate is stated is disputed among grammarians. The dispute is about grammatical government, not whether both men depart.
First establish whether the coordination links individual nouns or complete clauses. Semantic participation alone never resolves the question of case.