Is "who are doing the dishes?" an acceptable question?
It's an inconsistency in the language that both "Mary is doing the dishes" and "Mary and John are doing the dishes" become "Who is doing the dishes?" I presume this is because the person asking is nearly always completely ignorant of the answer (the number of people doing the dishes; his, her, or their names). To a native, "who are" sounds wrong.
On the other hand, if you know two or more people are doing the dishes, but you don't know which two, you could ask, "Which people are doing the dishes?" In this case you are requesting someone specify / add information.