I’d like to see if there’s an answer to this one too. In the past I’ve split the table into two different tables referencing the same model and placed them alongside each other in panel columns. Above these I’ve placed templates text with the span headers.

Louis model is the only one that would be available declaratively. 

You could use Javascript to add a second TH line that spanned a certain number of colums,  but  your on your own there…