Lockscreen's display(s) should use the same wallpaper as the desktop's display(s). For multiple monitors, this means that on the lockscreen on display 1 should display the wallpaper of display 1. The same goes for display 2. This would increase Plasma's consistency. This is the default behaviour of GNOME and I think Mac, and Windows. Therefore, it is what most people are familiar with.
We could potentially implement it as an option.