Your contract should have a two page notice titled "Notice to Buyer and Seller of Buyer's Rights....". Take a look at this addendum. Among other things, it describes the buyer's rights in the event that the seller does not provide disclosures at the time you sign the contract. Most of us never pay attention to this language since disclosures are pretty much always provided when you write the contract. If disclosures are not provided, then this notice says that the buyer can rescind the contract and have the escrow deposit returned within 5 days of receipt of the disclosures.
So what does this mean to you? Not withstanding other issues with your contract, I believe that you are fully ratified even without the other seller's signature on the disclosures. However, assuming that you don't receive disclosures before settlement, I believe that you could void (rescind) the contract at anytime and argue that you never received disclosures as required. Your argument would be that since only one seller signed the disclosures, you did not receive completed disclosures. This may sound like a technicality, but it's definitely legit if the other owner is aware of issues with the home and hasn't disclosed them.
I believe that you are OK, but the sellers are at risk in this case. Of course, I'm not attorney so don't treat this comment as professional, legal advice. Run it by your settlement attorney to get the right answer.
I have handled many short sales and you need to know your rights going in as a buyer.
I reccommed to e-mail the honest agent and long time friend that lives local.
You will be so happy that you met him and have peace of mind. Tell him that Kevin in Florida sent you!
Short Sale negotiations are very perilous for the buyer because of the uncertainty.
The least you should have is a complete ratified contract
Long and Foster
Silver Spring/White Oak