1. Decide what background you want for Metro, and set this as your desktop background first.
2. Go to Start screen, open charms menu>Settings>personalise.
3. You should see the pic you chose for the desktop available for you to choose as a Metro background, so choose it.
4. Go back to desktop, and choose the pic you want for your desktop.
Bit long-winded, and there might be a better way, but this seems to work.
On a similar issue (but kinda the opposite lol) I want to have the same wallpaper on my Surface and my external monitor. But when I set my wallpaper to random, it will display a different, random image on each screen...
No, RT. And ive tried setting Metro first and second and it only displays one.
If i set it as Desktop, then its set for Metro, but only available through the charms menu while its set as Desktop.
If i set as Metro and go back and click/hold on Desktop and choose another background (like if i had 7 or 8 installed), it still changes Metro, but then the previous image is no longer available in the charms menu.
Im not sure why any picture on your device is not available for Metro regardless of which image is current. So if theres a way to do it, then i cant find it, and im no dummy when it comes to Windows, although not as versed in Win8+ as i am in XP/7.