The official answer to Is it possible to post questions across a couple of the sites because it's debatable which site it fits best? is:
Allowing cross-posting is a slippery slope.
If you might have slightly better odds of getting an answer by posting it on two sites, well, by gum, why not maximize your odds by posting it on twenty sites!
There are some questions which fall into grey areas between sites, and I think it's OK to ask and delete, then re-ask if you feel you have asked on the wrong site.
But as a general rule, do not cross-post questions, please. Pick a site and go with it.
If you regret your initial choice and want to change sites, I recommend flagging for migration rather than deleting and re-asking, in case someone has seen the question and is preparing an answer.
Unix and Linux's FAQ explicitly says not to cross-post:
Cross-posting is strongly discouraged -- if you post on one site and then change your mind it can always be migrated to another. If you're not sure if your question is on-topic, ask on meta or just give it a try and the community will decide.
So Do not post the same question on both Ask Ubuntu and Unix & Linux. When a question is cross-posted to U&L and other sites, what normally happens is that all but one instance of the question are deleted. You get a warning the first time, and doing it routinely would be considered abuse.
By the way, althought the term “cross-post” is often used, there is technically no cross-posting facility on Stack Exchange. A given question exists on a single site; if you post questions with identical contents, that's a multi-post.
