This is my 2nd post here. I've looked everywhere but can't find way to put the same background on my second monitor. Only the active monitor gets the background.
I guess the solution was too easy for me. It didn't occur to me to try to just put the background on the 2nd screen separately. Out of habit I thought that choosing the wallpaper on the primary screen would apply it to all the monitors. So if someone else has the same 'issue'.
Just right click on the 'dark' (=backgroundless) monitor