You do have the option to submit in writing that you reserve the right to submit additional offer while the lender is reviewing your offer as well as provide the lender with an expiration of when you wish to have a response or you will withdraw your offer. Not sure if the lender will care about the expiration date, but you can try it.
I typically recommend that the buyer and seller both commit to each other in writing that the buyer will put earnest money up non-refundable for 60 days and in turn the seller will work only with your offer and will hold any others as back up only.
Best of luck!
If you can show that you can afford both of them, then you don't have a problem. If you can't afford both of them, offer on the one you like best. If both offers go through, you may find yourself on the hook for both homes.
Even if one goes through and you have a chance to back out on the other, I have to question the ethics of putting offers on two seperate short-sales at the same time. When you make an offer on a shortsale, it most likely gets listed as contingent and buyers in the neighborhood won't get a chance to see it. Not only that, you are hurting the owner's chances of selling their home. When a person is trying to short-sale, they are trying to avoid going into foreclosure. Time is of the essence for them and it could be a very nerve recking experience. It's not fair to the seller who loses in the end because if in 2 months time one comes back before the other and you take it, the other seller just lost two months in which another buyer could have come along. Pick the one you like best, and go for that one. It's the right thing to do.