A long time ago I was using 2 1680x1050's and it was easy to find backgrounds that worked because of the identical resolutions. I've finally decided to get around to fixing my wallpaper which has been broken since I purchased a monitor with a larger resolution.
I can't seem to find ANY wall papers that I can use. I guess the required resolution would be 3600x1080 (I'm okay with a bit of it getting chopped off on the second monitor). The choices seem slim.
I get all my dual screen wallpapers from this, many of them look amazing. On the left there's a filter so you can select your resolution (3840x1080 for you).