In KDE 3.x having a background span over multiple monitors was easy/possible. In KDE 4.x is does not seem to be possible.

Related bugs:
https://bugs.kde.org/show_bug.cgi?id=209504
https://bugs.kde.org/show_bug.cgi?id=227614

If anyone knows how to do so that would be great, otherwise I'll just have hope it gets added back.

Subscribing.

One way of working around this is to use an image editor to split the image appropriately, then place the appropriate sections as each screens background.

I do not think the Plasma Developers intend on providing this capability, although if they implemented the workaround method ( unlikely ) it might be possible. A Plasma script would probably be best to accomplish automating this.

Currently this would be quite strange, since the two activities ("desktops") on the two monitors are independent. I think the Plasma developers plan to make them linked, i.e. switching activity on one monitor switches the other too[1].

With the new behavior it might be possible to create a wallpaper type that automatically does what bcooksley describes. Don't take my words as the truth now, I'm just speculating.

[1] http://chani.wordpress.com/2010/03/07/a ... ext-again/

Developers have repeatedly stated they will absolutely not implemented containments spanning multiple screens. It works well enough when the two screens are the same size and at the same height and rotation, but in any other situation it leads to a lot of problems that the developers wanted to avoid.