The basic danger with having two offers out there is that they both could be accepted. Yes, you can put language into the second agreement but as a listing agent I would be hard pressed to advise a seller to take it and do all the work involved on a "maybe" offer.
Best to cancel the first offer and proceed with the second. And remember that all must be completely above board. If you have two offers out there, it is a material fact and must be fully disclosed.
Steele V. Propp
Bank Owned Homes and Short Sale Division
The documents, purchase agreement, signed indicated your intent, should the bank agree to original terms and the time frame doesn't exceed the agreed upon time line you would be obligated to purchase. If you put in a second offer on an other house with out expressing contingent to removal of prior purchase agreement and that agreement is accepted and you financially qualify you would be obligated to proceed. Last page of the standard purchase agreements states specially that this is a legal and binding contract.
What is the end date for the short sale addendum? Once that has expired you are no longer obligated to proceed unless other wise agreed to. If you want to offer on an other property make that offer subject to cancellation of previous contract.
Talk with your Realtor as they will have the best insight as to what to expect, time frame and options.
Some buyers will make 4-5 offers and make them all subject to the cancellation of the others - it's possible but some sellers will not want to be your 4th or 5th choice and will hold out for a buyer that will stick with them.