What is the subject in «الزَّيْدَانِ ضَرَبَا»?

Rules of the Fāʿil: Explicit Nouns, Pronouns, Number and Gender

• الزَّيْدَانِ as the verb’s subject
• The dual alif
• The mubtadaʾ itself is the verb’s subject
• The concealed pronoun هُوَ