Technical SMW forms note:

It is possible to use forms to store an identical parameter value in more than one template. This is useful for when two template want to access the same value and want to get the most recent value without double flushing. For example, both a person infobox and a child infobox needs to access the name of the parent. By refering to both templates on the form, and using the field in both for template areas, the value will be recorded in both templates.