Entity References Auto create between Nodes
You can implement hook_entity_insert() 


  Act on entities when inserted.
  
  Parameters
  
  $entity: The entity object.
  
  $type: The type of entity being inserted (i.e. node, user, comment).


and inside the hook check if you have more than one node in the reference field, in this case you will edit all the nodes and add the reference to the others.