When who and what are used to ask for the subject of a clause, they most often have singular verbs, even if the question expects a plural answer.
Practical English Usage, Michael Swan, Third edition, Page 525
So we say:
A: Who is speaking?
Q: John and David are speaking.
How about this one? Is it correct to ask:
A: Who is speaking with each other?
For this answer:
Q: John and David are speaking with each other.
Best Answer
"Who is speaking with each other" does not sound right to me; unfortunately, I don't have a source to cite to back this up. I wouldn't recommend "Who are speaking with each other" either though.
I would recommend avoiding using "who" as the subject of questions including the phrase "each other," and instead using something more specific if you can, like "which [plural noun]." "Which of them are speaking with each other?" is certainly grammatical.