If you make an offer on a short sale and then find a better home that you want, all you have to do is send a written Cancellation of Offer form to officially cancel your offer. Then your escrow deposit should be returned to you from the Real Estate company, title company, or attorney's office, whoever is holding your deposit. When you have signed and sent the Cancellation form, you are then free to make another offer.
I am cautious about making offers on more than one house at a time, however, because if both offers were accepted, then you would lose your deposit on one, or even be sued for specific performance. You might be able to cancel on the basis of the inspection since short sales are almost always sold "as is" with right to inspect, but you are taking a chance that you would not be able to show repairs required by a professional inspector. This depends on the wording of the Purchase and Sale Contract, of course. Anyway, we agree with your Realtor, it is still risky to place more than one offer at a time. Good luck