Ok,

When using multiple FieldLayouts a non-hierarchical list of objects one would expect the resulting visual structure to be similar to the use of multiple instances of the XamDataGrid control - showing different FieldLayouts as 'peers' in the same list.  It appears that in actuality, the first FieldLayout is being used as a "parent" of sorts as the column headers for this first fieldlayout are the only headers that appear without the expander to the left and the grand summary row is rendered below.  The problem here is that it creates the illusion that all rows from all bound objects are being considered in the grand summary, when in fact it is only the rows from the first FieldLayout.

I have defined a XamDataGrid with 3 FieldLayouts.  Each fieldLayout is being selected appropriately when the grid is bound to a heterogenous collection of objects although notice the position of the grand summary and the style of the header. 

1) Is there any way to render these objects as "peers"?  Have the column headers be treated the same?

2) Is there any way to not render the grand summary but keep the summaries for the individual groups?