Is there any way to stretch or span a wallpaper on kde plasma? I usually span this wallpaper, and I'm going to burst a vein if I can't span it!
It doesn't look like you can do this automatically in Plasma 5 yet. Your best bet would be to open GIMP and create 2 images that match the resolution of your monitors.
For example: I have two 1920x1080 monitors so I take a 3840x1080 image and split it in half and set both monitors separately.
Here is a bash script that will split the image for you.