Why was the word "لحكمهم" (for their judgment) in the plural form used instead of "لحكمهما" (for the judgment of both of them) in the dual form in verse 78 of Surat Al-Anbiya, even though the discussion is about David and Solomon, and they are two?

The Almighty's saying, "And We were witnesses to their judgment," with the plural pronoun "their," even though the intended individuals are David and Solomon, peace be upon them both, indicates that the minimum for a plural is two. This is the view held by some scholars. Similar instances have appeared in the Qur'an, Hadith, and Arabic discourse.