The official answer is that simultaneous submissions are fine; just be aware that, as of the moment, any given author will only have a max of two stories published in any given year (although the serials are an exception to this rule).
The unofficial, advice-laden answer? It's better to write one story, submit it, and see how that is received first before considering sending another. For example, what do you think would be more discouraging, especially to a new author: (a) a single rejection on a story, possibly with some advice on how to improve, or (b) five rejections on five stories right in a row, since they all have the same pitfalls in terms of characterization, storytelling, grammar, etc.?