As near as I can see you can only add Facts to this report, using the far-left "items to include" button on the button bar. So, I don't believe you can add parents of spouses unless you added parents of spouses as Custom Facts.
You can devise a Custom Report that includes this info. Spouse's name and Spouses' parents names are listed in the Facts List for the Custom Report.
You can devise a Custom Report that includes this info. Spouse's name and Spouses' parents names are listed in the Facts List for the Custom Report.