You could also create an XML schema, create a data source based on that schema then bind fields on both sides of the form to the same schema element. Editing either field edits both fields.
It sounds overwheleming, but creating the schema is pretty easy since you'll just copy and paste the same line over and over then change the element name. Creating a schema as the first step of any new form will save you headaches later. You then have a data heirarchy separate from page layout. If you move items around in and out of sub forms, the data heirarchy stays the same.
