Two Master-detail relationships on a new custom object is how an app builder should configure the data model to create a relationship between Contact and Solar Project objects where each Contact can be related to multiple Solar Project objects, and each Solar Project can have multiple Contacts associated with it. This is an example of a many-to-many relationship that requires a junction object with two master-detail relationships. One Master-detail relationship on Contact and one Master-detail relationship on Solar Project, two Lookup relationships on a new custom object, and one Lookup relationship on Contact and one Lookup relationship on Solar Project are not valid or correct ways to create a many-to-many relationship.