If you’ve found a likely market for your story, temptation is strong to send them everything you have that is finished and a possible fit. That way, they get to pick from your stories, and if you strike out with one, they may like another.
That is, unless the guidelines state that they accept “multiple submissions”. Most editors don’t: they want to see one of your stories at a time, and will look at another of your tales only after they’ve dealt with the first. If they don’t explicity state that they accept multiple subs, they will most likely reject your stories instantly if they slide out of the envelope—or come attached in an e-mail—in pairs, or worse, threes or fours.
Even more of a faux-pas is to send your story to multiple markets at once, the so-called “simultaneous submission”. Unless the guidelines (of every market you’re considering doing this to) specifically state that you can, you should never do this.
Don’t get me wrong; I understand the temptation. You’re in for a long wait with a high probability of eventual rejection; the temptation is to hedge your bets. But imagine sending your story to both Analog and Asimov’s. Imagine that the Analog editor gets around to your story, reads it, and loves it. Imagine that he then reads it a second time, spends some time on preliminary editing, shares it with a colleague, and then sends you an acceptance letter. Imagine that you’ve already had an acceptance letter from Asimov’s a week before. Imagine having to tell the editor of one of the highest-paying, highest-kudos, most rarified publications in genre that the story you’ve submitted to him, and that he really, really wants to buy for his magazine, is already sold to someone else.
No, this is not the beginning of a profitable bidding war. This is the end of your relationship with the Analog editor, and if you’ve contacted the Asimov’s editor to play out this new development to your advantage, probably also the end of their publication offer. The two editors will meet up at the next Con and shake their heads at so much unprofessional stupidity as they raise their glasses. Other editors will enquire and learn about your error. You will become a household name in genre, not for your writing qualities, but for your record of unprofessional and unreliable conduct.
But by all means, if you can’t help yourself, play the odds and send out simultaneous submissions. Just don’t say I didn’t warn you!