Even if I stretch, or center, I get two images on each monitor rather than one image across my two monitors. See image. What's going on? Attached is a screen capture, scaled down to fit the ridiculous upload limit of 256k.
I feel like the app is doing the right thing, and properly representing how you have it set up but Windows is acting differently and using different portions of the image and tiling it.
Maybe the app is having trouble setting the wallpaper 'fit'.
Which version of Windows are you using?
Not using any other wallpaper apps?
If you set your left screen to primary does that resolve the issue?
Also if you can navigate to "%appdata%/MultiWall/" and find the file Layout.json and upload it here, we can see if the layout is getting messed up.