Last July I was in Stacy Hyde in Dallas and was taken with the simple gesture of hanging a mirror over a mirror. The addition of this convex sunburst mirror adds interest and texture to what is otherwise a standard.
Here DC based designer Kelley Proxmire hangs a large, round convex mirror over a mirrored wall, reflecting light and expanding the space. This dining room was part of an Eastern Shore Decorator Showhouse from a few years ago. I happened to go to this showhouse and loved this room. The simple centerpiece would be great for a Thanksgiving table setting. For more - go to Kelley Proxmire's website here.
The mirrors in these two pictures are hung over antiqued mirror for a nice effect. (above Elliott Puckette's home, American Vogue, May 2003, photography by Francois Halard via Style Court)
Home of Dana Slatkin, California Style Magazine, Jan/Feb 08 via Girl Meets Glamour
Tricia Huntley layers a Venetian mirror for a glamorous, custom look. (photography Kevin Allen)

After spending an inordinate amount of time searching for more pictures of mirrors over mirrors and running dry, I wondered about layering Art over a Mirror?
What about hanging a print that is framed with a surround of glass (like below) over a mirror? You could use ribbon to hang it - maybe even doing something just for the holidays... (pictures from my visit to And George in Charlottesville, VA)
Or, as Kelley Proxmire did here, hanging a Mirror over a Window?
(image from Kelley Interior Design here)

If you're interested in more on layering mirrors, go to Apartment Therapy DC here. I discovered Leah did a terrific post after I stayed up way too late looking for more pics!