Yes. In this situation, the second offer, once accepted, can be a back-up contract only. According to Paragraph B of the Addendum for Sale of Other Property by Buyer, the seller may not compel the first buyer to waive the contingency or terminate the contract under the addendum until the seller accepts a written offer to sell the property.
The seller may not accept a second offer unless the back-up addendum is part of the second offer. Otherwise, the seller may be obligated to sell to two different buyers, especially if the first buyer waives the contingency.