In some cases the lender MAY treat yours like a right of first refusal givin you the opportunity to match the the offer but many do not. Some agents put in a clasue in the short sale clauses they get also get a right of first refusal if another offer comes in but sophisticated lsiting agents counter them out. Allowing a term like this to pass is a deterrent to backup or secondary offers.
Once you get into contract though, your power is the sellers signature on your contract. If the seller receives a backup offer that is better (and the likelyhood that the lender will kick your offer for the better one) your agent has to go to work to cajole the othe buyers number out the the listing agent (which he is under no duty to disclose to you) and give you the opportunity to raise your offer before your time runs out.
This is reason 427 why short sales are such a challenge.