Francisco,
For a good clean design where you have a well balanced load of responsibilities, business objects should *normally* be responsible for updating a single table on the back end. That said, business objects can definitely present an unnormalized view of data, and there are circumstances where a business object can update more than one table...if I understand your example properly, it sounds like this is a case where you may want the business object to update both tables.
Regards,
Kevin McNeish
Eight-Time .NET MVP
VFP and iOS Author, Speaker & Trainer
Oak Leaf Enterprises, Inc.
Chief Architect, MM Framework
http://www.oakleafsd.com