Something to note:
When you send your manuscript to a journal, the editors want to get the best and most appropriate reviewers for your manuscript. Chances are, these people are asked by more than one journal to submit reviews and if you have sent your manuscript to more than one journal simultaneously, the odds are not so unrealistic that your manuscript is sent to the same person from two different journals. Not good.
When a journal is making a decision whether to review your manuscript or not in the first place, they have many factors to consider, ie space issues in the journal, copyflow issues from the publisher, and content issues in terms of appropriateness for certain issues. So, if you send your paper and two journals accept it, chances are that both of them have already taken alot of other issues into account to accept your manuscript and will be pretty upset with you wasting their time. Not to mention if you ever want the journal to consider another manuscript from your group again--forget it!
Also, if you are resubmitting a manuscript to a different journal after having been rejected from a different one, don't forget to reformat. If you just send it in the wrong format for the journal you're trying out now, they will know that it was rejected from somewhere else and that does not sit favourably with the editors.