I find it sort of odd that Microsoft didn't provide a good OOTB way to keep the metro and desktop backgrounds in sync. The abrupt switch from desktop to metro is a bit hard on the eyes with the default settings. A quick tip to make this more pleasant is to make your classic desktop a solid color that matches that of the metro desktop. I like grey so in my example I setup metro the way I wanted it and then took a screenshot, opened it in paint, and then using the eye dropper tool got the RGB value. I then put this into the desktop solid color background and now I have a much nicer transition between metro/not metro. The grey color is R: 88 G: 88 B: 88 in case you're curious.
Here is what a screenshot looks like on my multi-monitor setup with metro next to classic.
UPDATE: This capability will be built in to Windows 8.1