There are two proposals on Area51 which I am interested in, particularly because I believe that they provide a nice way of segmenting a field. Would it be acceptable to post the same questions to both to illustrate or test the delineation between the two? And is there a way I should indicate the duplication?
If it's on-topic (or blatantly off-topic) on each proposal: Sure, absolutely!
I'd say "no" in the defining stage of a site, which is all about finding out which questions are suitable and which are not. Whether a question is featured on another site is meaningless at this point.
"Yes" on a live site to avoid pissing off people who take the time to actually answer your question.