First, put the directory into “Details” view mode (if it isn’t already)
so you have column headings.
If you click on the “Date Modified” heading, which will sort by Date Modified (descending), and then Shift+click on “Name”, it should remain sorted by Date Modified (descending), but with the folders at the top.
Note: This is observed as not working while the “preview pane” is enabled (Windows 7).
Simply disable the preview pane temporarily with Alt+P before sorting.
The preview pane can then be re-enabled and the sort method will remain.
To revert this, just click on whatever column you want to sort by.
Once you have the sort order that you want,
you can switch into one of the other view modes
(e.g., “Icons”, “List” or “Tiles”),
and the presentation will retain the sort order that you specified.