Many-To-Many Relationship issue

Hi everyone. I’m using SugarCRM to manage some things in our company, Properties and Owners. An owner can have a Role in that property like Owner, Guarantor. When I assing a Property to an Owner the first time it works well, but, if i try to add the same Property to another Owner, it replaces the previous relationship in the middle table. Another issue is that I cant figure out how to add a custom field to the table between relationships.

Any help is appreciated. Thanks.