Why does the plural form "قالوا" and the dual form "خصمان" appear in the verses about the two disputants who came in upon David?

The verse mentioned in the story of David (peace be upon him) begins with the Almighty's saying: {And has there come to you the news of the disputants, when they scaled the wall of the prayer chamber - when they entered upon David, and he was frightened by them? They said, "Fear not. [We are] two disputants, one of whom has wronged the other, so judge between us with truth and do not deviate, and guide us to the sound way."} (Saad: 22). The verbs (تَسَوَّرُوا - tasawwarū - they scaled, دَخَلُوا - dakhalū - they entered) are in the plural, even though only two individuals are being referred to. This is because the word "الخصم" (al-khaṣm - disputant/adversary) can refer to a singular, dual, or plural entity, and can be masculine or feminine. The plural form here indicates the joining of one thing to another. Such usage has appeared in the Holy Quran and in Arab speech, as in the Almighty's saying: {These are two adversaries who have disputed} (Al-Hajj: 19).