Should unanswerable questions be downvoted as these have no answer? For instance Can two people master the Elder Wand at one time?
No. Not for the reason described.
As discussed in the earlier question Should all questions without explicit canon answers be closed?, the lack of a canon answer does not make a question invalid or bad.
I, myself, tend to only downvote questions for the reasons described in the hover-text:
- The question does not show any research effort
- The question is not clear
- The question is not useful (I don't really feel like this reason applies to many SE sites)
The lack of a canon answer does not fulfill any of those three criteria and does not deserve a downvote, in my opinion.
For this specific question, one of the arguments is that it is a manufactured what-if scenario. While it is an unlikely scenario, it is not a Shark Vs Gorilla question, nor is it out of the realm of possibility (at least, based on my understanding of the rules/physics of the HP universe). The lack of canon answer leaves us with logical speculative answers, which are perfectly fine.
For the record, I am similar to @Beofett: your question wasn't woth an upvote as is BUT on the other hand people who downvoted it (and dissed it in comments) were within the letter their legal rights but clearly abused the spirit of downvoting guidelines).
First of all, because the guidelines for downvoting clearly do NOT anywhere state that answerability of the question is grounds for downvote. Lack of research, lack of clarity - yes.
Second, because absent a specific proof of non-answerability (which would be tremendously difficult to prov in general; and impossible to prove for HP question as per my point #3 below); an accusation of "not answerable" is a subjective opinion not backed up by facts.
To add to @Beofett's answer, to make your question worth of upvote, I would recommend explaining WHY such a situation would be plausible in-universe.
Did something happen where 2 people disarmed/defeated (or tried to) the same person at the same time in the books? BAM! Your question goes from "speculative" to "interesting" and IMHO worth an upvote.
To address "unansweable" BS stated in the comments on your question: Since the author is still alive and answers questions, calling your question "unanswerable" is wrong, incorrect, illogical and fallacious, even leaving aside the generic issue of what votes should "really" unanswerable questions get.
TL;DR I wouldn't have downvoted your question, nor would I have voted to close (but I probably wouldn't have upvoted it, either).
I'm going to split my full answer into two parts.
For the first part, I'm just going to comment on the "should x be downvoted": Voting, whether up or down (or not at all), is entirely the prerogative of the individual (subject to reason, of course; targeted voting, for or against another specific person, is against the rules).
We, as a community, have no control over whether any given individual upvotes or downvotes a question or answer. We can set a clear policy, but we can't enforce it, so what's the point? This is "by design", since it should be majority opinion that is the indication of quality, so if one person (or even a handful) downvotes what a lot of people are up-voting, it won't mean much in the end.
That doesn't mean downvotes should be cast willy-nilly. Receiving downvotes is generally very discouraging, particularly to new members of our community, and should, in my opinion, be done both sparingly and with a clear indication as to why it was given. But again, we can't enforce that. We can only suggest.
Now, for the second part, let's address the "can't be answered" part.
No, I don't think just because a question doesn't seem to have an answer within the primary source means it should be downvoted or closed. In particular, looking at the comments, I have to question the justifications you were given.
The users downvoting you and commenting seem to be under the impression that the Harry Potter books are the only source of Harry Potter canon. That simply is not true, and even a cursory review of the many Harry Potter questions we have on our site will reveal that in addition to the Pottermore site (which is, I believe, considered supplemental canon in its own rights), J.K. Rowling has provided quite a lot of additional material and information about the Harry Potter universe than is contained in the books.
Direct quotes from an author during an interview are certainly "canon" as far as I'm concerned, and the general consensus of the community seems to agree with me.
The closest thing I could see to an appropriate criticism to your question would be "primarily opinion based", which is a close reason, not a downvote reason. However, that's a rather slippery close reason, and I don't think it is at all appropriate in a case like this, where you're discussing specific details of a single, specific work, with a hypothetical situation thrown in. You can't know if something like that can only be answered by opinion unless you've done extensive research, and even then it is easy to miss something authoritative (especially in a universe like the HP setting, due to the high volume of supplemental materials).
To me, "primarily opinion based" is generally for when no specific work is included when discussing a hypothetical (and even then, only sometimes), or when multiple specific, yet unrelated works, are involved in hypothetical discussion (and again, only sometimes).
Ultimately there's no such thing as an unanswerable question unless;
- You're the author and sole canon source of information
- You've never shared the answer with anyone
- You plan to kill yourself immediately after writing "no, there's no answer"
If any of the above isn't the case, there's always the possibility that an unknown source will come forward with more information or the author will extend the canon with future works that give more details.
In the specific case of your question, the downvotes seem to be because you're specifically calling for speculation about a possible event even though the answer (e.g. "no, that can't happen") is already well established in other questions.