Depending on the way that the two Sellers held title to the property, and depending on the laws of the state where the property is located, the contract to sell the property should still be enforceable by the Buyer.
If the contract becomes entangled in probate or transfer, the Buyer may need to pursue action either against the surviving Seller (if he/she then owns the property in his/her sole name after the death of the other owner/seller) or against the estate of the deceased Seller.
But be advised, this is general information only, as many specific facts would be involved in a situation such as the one presented. Such a circumstance would likely require the Buyer to obtain legal advice from a qualified professional.