"A new publisher?" I asked. "Did X reject it?" (Because I was surprised that her previous publisher would even consider letting her go.)
"Well," she replied, "I never heard back from them on it, or from my other publisher, and it's been nearly two months, so I thought it was probably time to send it out again to somebody new."
My face must have revealed my shock, because she said, "What?"
"You've sent the same manuscript to three publishers? At the same time?"
"Well, yeah. What's wrong with that?"
The answer is that simultaneous submissions are a very very bad idea. It's like trying to sell the same car to three different buyers. It can get you into a lot of trouble.
Most publishers are very explicit on their websites or in their submission guidelines that they do not accept simultaneous submissions. (There are a few--very few--exceptions, that explicitly accept simultaneous submissions, but even they insist that you tell them it is a simultaneous submission.)
The publishing industry is filled with stories of editors who, finding that this or that title missed a crucial deadline or otherwise isn't ready to go to press, reached down into the waiting pile of submissions, pulled out the next title in line, and fast tracked the editing, artwork, book design, etc etc, and then sent off the good news to the author that their manuscript is not only accepted but already halfway to press -- only to find that the author has just sold the manuscript to another publisher. (Yes, yes, smart editors would not order artwork before they had a signed contract in hand, but it has happened often enough to be the stuff of legends.) Needless to say, publishers get very pissed when they find themselves holding a cover to some other publisher's book; and that that author need never cross their doorway again--and probably shouldn't bother trying to submit to any other publisher that publisher has ever had lunch with, either. There are a million manuscripts out there awaiting publication, a thousand as good as the best, so publishers will often simply decide not to bother with an author they have come to think of as 'unreliable'.
But forget about the anecdotes of books halfway to press before the publisher discovered they didn't in fact own the rights. More commonly, the problem with simultaneous submissions is simply that you are asking the editor to invest 8 to 10 hours reading a manuscript you may not end up selling them. Editors are busy people who already have more manuscripts on their desk than they can possibly read before quitting time, so if they push other manuscripts aside to read yours, you're not just cheating the editor out of his time, you're cheating other writers out of their shot at that editor.
Even in small one or two person presses, it can take two to three months to decide on a manuscript. Admittedly, the first two months are probably spent with the manuscript sitting on a pile of other unread manuscripts while the editor frantically attends to some other more immediate deadline; or just attends to the stack of submissions from two months before that. But even once your manuscript gets that first read, the editor has to weigh it against other submissions, maybe talk it over with another person at the press, maybe play around with how this or that element could be 'fixed', and so on, before a decision can be made. So it is unlikely that you would hear back before at least a couple of months go by. Check the publisher's submission guidelines to see if they have mentioned their response times. If it says 8 to 16 weeks response time, that's how long you should expect to wait. Throw in another couple of weeks for good measure, then send a polite inquiry about the status of your manuscript. If there is no response, or time keeps going by and they don't seem to be making a decision, you can always withdraw the submission and take it elsewhere. Taking it elsewhere without first officially withdrawing it can place the editor in a very tricky situation.
For example, at larger presses, there is typically an editorial meeting at which each editor brings forward her three or four nominees and has to argue for them against the nominees of the other editors. This doesn't just represent the further investment of time in your manuscript, but may actually matter to the editor's career. If they've championed your manuscript at the meeting, but have to go back to the next meeting to confess that your's has been withdrawn because it has already sold elsewhere, than that can represent a significant loss of face for your editor. The other editors will be pissed that their nominees were trumped by a manuscript your editor was not in the end able to deliver. The stakes are similarly increased each time your editor has to defend your manuscript as the decision to purchase is vetted by the marketing department, more senior editors, the editorial board, or the publisher. So editors come to loath simultaneous submissions, even if they haven't invested a dime in production.
So, although it feels like the press should be able to respond immediately, or that they must not be interested if they do not respond within a few weeks, deciding whether to publish your book is a time consuming process. You must therefore give the publisher time to finish the process before taking your manuscript to the next publisher.
Another factor that this particular author had overlooked was that she had previously published with two of these publishers. That certainly gives her a leg up on newcomers submitting to the slush pile, and increased her expectations of a speedy response, but it also greatly increases her responsibility to the publisher. That publisher has already invested in developing her manuscript/talent, in promoting her writing, and so on, so that unless there have been significant problems with the experience, the writer should consider giving that publisher first right of refusal. Indeed, many first book contracts include a clause that make that a legal requirement. Submitting the book simultaneously to the competition may actually leave an author open to suit for breach of contract. (And again, the second publisher will not be overjoyed to find that they are being sued by the first publisher because you submitted a manuscript to them that you shouldn't have.
So basic rule: unless both the publishers to whom you are submitting explicitly say that simultaneous submissions are okay, don't even think about doing it. Allow each publisher in turn sufficient time to make a determination about your manuscript before withdrawing it and moving on to the next. Spend your time working on your next manuscript, rather than obsessing about how long its taking to hear back about this one.