Considering other offers is a seller's decision and any additional offers they decide to accept, would be in back up position to the first accepted offer. Have your Realtor contact the listing agent to find out if they are accepting back up offers. If you don't have a Realtor, then you can contact the listing agent directly.
If the sellers are willing to accept your offer as a back up, your offer would remain with the selling agent until the first buyer walks or the offer is declined by the short sale lender/servicer. Short sale lenders/servicers generally don't want additional offers sent to them, because they can only work with one offer at a time.
If the sellers are willing, and you decide to write a back-up offer, you should make sure that language, giving you "an out" if you find another house, is included in the offer. There are Realtors who will write additional offers, on other properties, even after their client's offer has been accepted by a short seller. This could cause legal problems. Technically, once an offer is accepted by the seller, the buyer is legally obligated to stick with it until the date they have given on the short sale addendum has passed.
Short sales are a relatively new frontier for most Realtors and for a while there it was like the "wild wild west," where the rules were not clearly defined or understood. In the recent past, it wasn't unusual, but it was illegal, for buyers to have more than one accepted short sale offer out there.
Make sure you are protected and not locked into an offer that may sit in a file on the listing agent's desk for months, only to be tossed because the first offer was accepted by the short sale lender/servicer.