On Ubuntu 18.04.1 a multiscreen screen lock wallpaper works using Tweaks (gnome-tweak-tool). Open Tweaks and in the Appearance tab in the Lock Screen section, select your multiscreen wallpaper as the Image and set the Adjustment to Wallpaper.
On Ubuntu 18.04.1 a multiscreen screen lock wallpaper works using Tweaks (gnome-tweak-tool). Open Tweaks and in the Appearance tab in the Lock Screen section, select your multiscreen wallpaper as the Image and set the Adjustment to Wallpaper.