Check the spelling and punctuation. Spell check programs are very useful but be careful. They may insert words similar to the word you have chosen simply because they do not recognise the original. If your manuscript looks shoddy, some Editors may not bother reading it at all so at all times be professional . Multiple submissions are generally discouraged.
Some writers submit their work to various publishers simultaneously.
If you do this, you should let the Editors know. As a rule, this is done more frequently with novels tahn short stories. Many magazines will blacklist you if they find out you have been sending the same of piece of work to their rivals at the same time. At some point during, or perhaps after the first draft has been written, you will need to revise it. Be objective and be ruthless.