This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
One line of thought: Don't. Yes, editors keep
stories for far too long, and yes, it isn't fair that they
can waste months of your time without leaving you anything
to show for it. However, following the rules is the best
way to make certain that your manuscript is read.
Another line of thought: Do it. The chances of
having two editors accept your work at the same time is so
remote as to be almost impossible (although it has
A possible workaround: Submit works with a time
limit; say in the cover letter that if you have not received
a response by three months after the date of submission, you
will withdraw the work from consideration and will resubmit
the work elsewhere.
If you decide to simsub (send simultaneous
submissions,) be honest and mark the submission as
simultaneous. If you get caught simsubbing without noting
it on your manuscript or cover letter, your name will be mud
(and remembered, and passed on to other editors.)
The _Writer's Market_ and the publishers' guidelines
will say which magazines/book publishers accept simultaneous