Why Do 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.
Pros and Cons of Simultaneous Submissions for WritersObviously 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.
How Do You Know?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. Novel & Short Story Writer's Market 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.
For more information on the publishing process, see the guide to getting published.