Simultaneous Submissions for Literary Magazines
A simultaneous submission is the submission of the same short story or another piece of writing to more than one literary magazine or publisher at the same time. When submitting work, it is important to check whether or not the publication accepts simultaneous submissions.
Why Some Journals Prohibit Simultaneous Submissions
When editors put an issue of a literary journal together, they're thinking about how all the stories and poems will work with each other to essentially create a book. It makes their lives more difficult if they've planned an issue to include a certain story, and then find out from the author that they've already promised it to someone else. Additionally, if a reader is excited about discovering a new piece and can't wait to get it out to the world, finding out that they will not be the one to publish it (and that another literary magazine or journal will) can be extremely disappointing for an ambitious and talented editor.
Pros and Cons of Simultaneous Submissions for Writers
Obviously a story's chances of getting published go up if it's being seen by more people at the same time: so in general, we like simultaneous submissions. On the other hand, literary journals that don't accept simultaneous submissions do tend to respond more quickly. So you don't necessarily have to write a journal off just because they don't take them. You will just have to be more patient. However, if you are rejected from a magazine that does not accept simultaneous submissions, immediately send the piece out again.
How to Know If a Journal Accepts Simultaneous Submissions
Finding out whether a journal takes simultaneous submissions or not will be part of your research if you're planning to send a story out to a lot of journals and magazines at once. The lists of literary journals on this site include this information when it's available. The Novel & Short Story Writer's Market book also specifies which journals and magazines accept simultaneous submissions. For the most up-to-date information, of course, you may always check the journal's website.
If a magazine's information page does not mention simultaneous submissions at all, it is safe to assume that they do accept work that is being sent out to multiple magazines. It is good form, however, if your work is accepted at one magazine, to let the other magazines you have submitted to know that the piece is no longer available for publication. You can do this with a short note to the editors you sent the story too, mentioning where you will be published and that you hope that in the future you will be able to publish with them as well.