In my opinion questioners should ask their question on either ELU or ELL, but not both. My rationale is thus:
There is significant cross-over from ELL and ELU in terms of our answerers. Asking questions on both sites therefore doubles the work for answerers.
It is disheartening for an answerer to answer the question, only to see that their hard work answering was unnecessary, since the question has already been answered verbatim elsewhere. This wastes valuable answerer time.
Discussions, comments and so on that are attached to one question don't get attached to the other - making it harder for the two posts to remain consistent.
Once the question has been answered on both sites, there is a conflict - which is the better answer? The two sites could have entirely conflicting "preferred answers".
My suggestion then is this
Questions that are asked on ELU and again verbatim on ELL (whilst the question is still open on ELU) should be closed as "not constructive" immediately, unless the question clearly distinguishes why the asker would like to ask both communities.
For example, the following is OK:
Do you put a space before punctuation in English? I'm an English teacher and I need to know the basic answer for my students, but I'm also interested in a more in-depth answer about under what circumstances this "rule" can be broken as well.
ELL answer: No. Punctuation has no prefix space and one suffix space (except for open-parentheses which are one prefix, no suffix). Always put punctuation in the right places or your essays will come back with lots of red ink on them and you will get bad marks in exams.
ELU answer: As the ELL answer, but here are some interesting old fashioned bits of text showing that it used to be different, some talk about fonts and why double spaces after punctuation is generally discouraged, and perhaps a slight discourse on how some editors prefer the old way.
The following is not OK:
What does this sentence mean: "It's raining cats and dogs"?
ELU answer: It means it's raining a lot. BTW, why are you asking the same question on ELL?
ELL answer: It means it's raining a lot. BTW, someone on ELU has already answered your question.
On this question, vote up if you agree that we should close such questions, or vote down if you think we shouldn't.