//    List<List<Contact>>


Contact contactR1 = new Contact(LastName='Test1');
Contact contactR2 = new Contact(LastName='Test2');

List<Contact> contactList = new List<Contact>{
    contactR1,contactR2
};


insert contactList;

Relationship__c relation = new Relationship__c(
     Parent_Contact__c = contactR1.Id
    ,Child_Contact__c  = contactR2.Id
);

List<SObject> sobjectList = new List<SObject>{
    relation
};

insert sobjectList;