To understand how Addresses and Contacts are Required on Deals, click HERE to be directed to the Knowledgebase Article.
Below are some screenshots that will help you navigate through this process.
Screenshot below shows a Deal in an enforced Sales Stage, Advise, and the 2nd Contact has been disabled from being selected as it does not contain the required information while the first Contact does.
Screenshot below shows how the Mobile App has also been updated to include multiple contacts on a Deal. A new Contact view exists at the bottom of the Edit Header section on a Deal allowing you to select from the Contacts affiliated with he Account and/or Agency on the Deal.