Why is the story of Prophet Moses sometimes referred to in the singular and sometimes in the dual (he and Aaron)?

The pronouns referring to Moses (peace be upon him) in the Qur'an are singular when the address is directed to him alone, and dual when the address is directed to him and his brother. As for the Almighty's saying, "Indeed, we are the messenger of the Lord of the worlds," in the singular, the answer is that "messenger" here means the message, or that it is used for singular, dual, and plural, or that the intention is that each one of them is a messenger.