For conferences with an abstract deadline shortly before full paper deadline, does the "no simultaneous submission" rule include abstract submission? It's generally forbidden to submit the same paper at two different venues at the same time. In addition, many computer science conferences have two different submission deadlines: a deadline for the submission of abstracts, followed by another deadline (1-2 weeks later) for the submission of full papers. A complete submission consists of an abstract and the accompanying paper.

Let's say we have a situation that involves two conferences, A and B:


Conference A had its abstract and paper submission deadlines some time in
October. A's website says that acceptance notifications will be sent
to the authors on December 7.
Conference B has its abstract submission deadline on
December 1 and its paper submission deadline on December 8.


Is it OK to submit a paper to A and an abstract to B simultaneously, cancelling the submission to B in case the paper is accepted at A?

(It surely seems advisable to ask the conference chair of B, but since I know the chair of B personally, I don't feel comfortable asking, since this might come off as a request for favouritism.)
To answer this question, let's recall why many journals and conferences do not allow multiple submissions. So,


  Is it OK to submit a paper to A and an abstract to B simultaneously, cancelling the submission to B in case the paper is accepted at A?



Multiple submissions are beneficial for authors, since they generally increase the chances for a paper/talk to be accepted and/or reduce the time until acceptance. 
However, this strategy is not beneficial for journals/conferences, since each venue will spend time and resources on processing the submission (register and peer-review it), and only one journal/conference will eventually publish it. The efforts spent by reviewers of another journal/conference will be wasted.
This strategy is also damaging for the community of researchers. Firstly, peer-reviewers and editors are also academics, and they do not have much time to spare, so it is best not to waste it with pointless review process. Secondly, and more importantly, readers generally benefit from well-written and well-edited papers, and this relies on careful and time-demanding peer-review process. Faster acceptance may indicate better organised peer-review process, but it can also mean that the process is superficial (like it is in predator journals and such). By preferring a faster route, one can waste time of highly professional reviewers and also rob himself of a chance to get their valuable advice and improve their paper/talk accordingly.


So the answer is no - it is not OK.