Generally, a business object is mapped to a single table in the database. This allows you to have a separate business object for each table in the database. A business object can also be mapped to a view in the database. This allows you to create any composite schema within the database and create a business object that can query that database object and expose the data through strong-typed properties on the business object. The view does not have to be done through a one-to-one relationship, but can be literally any view you design. Since a business object's internal DataTable is just a plain-Jane DataTable, its structure is flexible and can contain the data retrieved from any database query (you don't have to have a strong-typed property for every column for which you retrieve data). If you do not have a strong-typed property for the column, you can always retrieve the value through the .Item (indexer) property of the business object by specifying the column name. Additionally, you can use those columns in list population, since the dropdown lists that allow you to select the column names also allow you to manually type the name of a column if it is not available within the list.
However, that being said, there are only two types of objects within the database that can be updated using a business object's Save() method: tables, and updatable views. A business object can update its corresponding table through dynamic SQL or through stored procedures. With an updatable view, you can only update the view with dynamic SQL if the view is indexed or if you use the said "instead of" triggers, but you can always update a view using stored procedures (since the business object doesn't really "update" the table, but just calls the stored procedures to do the work, and you put the necessary code within the stored procedures).
So, that's basically it... you can map a business object to a table or to a view, and you can update both of those however you want. So, as long as your relationship can be logically represented through a view, then you can use a single business object to update it.