There is lots of confusion as to how a short sale works, which is why a low percentage are successful. The bank instructs the seller to only accept a fair and market based price... whatever that is. The seller should then sign the offer and submit all the paperwork to the bank ( signed offer, hardship letter, financials, and a HUD statement that shows how much the bank will lose ). Only upon receipt of all of that will the bank spend the time and money to hire appraisers and brokers to assess the home to see if the price is fair.
If they can justify the price, they will sell it. Many agents just submit offers or unsigned contracts to the bank which does not work.
To answer your specific question.... once the seller has signed another offer, they are legally bound to that person. So unless the other buyer walks ( which is common in shortsales ) or the bank rejects the offer they cannot sign you offer.
Ask the sellers agent very specifcally, has an offer and/or purchase and sale contract been SIGNED. If it has, an offer can be held as a backup. If NOT, then any amount of talking around it ( we're negotiating, talking with the bank, we're working with a buyer now.. we have an offer on the table etc etc ) is irrelevant. Write up and present your offer.