If you bring up the file log viewer for a file (right-click in Explorer, select TortoiseHg - Revision History) you get a dialog that shows you a list of the file revisions. You can multi-select two revisions, but if you then select "Visual Diff File" you don't get a visual diff of those two revisions. It instead shows a diff one of just one revision.
So not only is there no way to diff between two different revisions, but the GUI is misleading in that it lets you multi-select lines implying that you will be viewing a diff of those two revisions. (If you're not going to support operations involving multiple revisions, then don't allow the user to select multiple revisions.)