In my understanding, the pronoun "who" as subject of the main verb is always singular, even if the question is about more than one individual, e.g., "Who is coming to the party?" "John is/John and Mary are."
Longman English Grammar says: However, plural verbs can occur quite naturally after subject questions with Who, Which and What:
Who are playing in the orchestra?
Who have won Nobel Prizes for literature in the past ten years?
If "Who are playing in the orchestra?" is acceptable, I wonder if the following expressions are acceptable, too.
1. "Who are coming to the party?"
Situation 1: I know that more than one person is coming.
Situation 2: The person who is having a party said to me, "John, Bob, and Susie are coming to the party," but I couldn't catch their names.
2. "Who are playing tennis in the yard"?
Situation: A friend of mine and I saw two boys playing tennis in the yard, and I did not know who they are.
I wouldn't use "are" in any of those examples. These types of questions need the singular.
A question like "Who are those guys on my lawn?" uses "are" because we know that "those guys" are plural. In a question that doesn't have a specific, known, plural target, use "is."