What is Mr. X and his father doing?
What are Mr. X and his father doing?
Which one of the above is correct?
My friend says that both are acceptable in accordance with the context in which they are use d.
In informal occasions, especially when we know both the subjects are doing the same work together, we can ask the question as in sentence 1. But in formal occasions, and when we know that they are doing two different things, we ask the question as in the sentence 2.
Is my friend correct?