set_skin_offset()
. So if we create a wallpaper sprite which is as double as high as our ui_size (in this case 2 x 608px) we can put 2 wallpapers into this one image file like shown on the left. The visible area is only from 0 pixel to 608 pixel.
We can now set a button for example which moves the background skin 608 pixel vertically to put the second wallpaper into the visible area. This short demo video shows how it works: