I will try. I use wallpaper for example that extends across two monitors. Its extended wallpaper for two monitors.
I will open up something like photoshop and a picture i want to edit. Dont maximize the picture. Shorten it. drag it to the other monitor. Then i will open another occurrence of the program. I can max that screen then edit the picture. If you max a screen like firefox then resize the screen, that program will always start on that screen. which ever screen you drag it too. If the screen is full size then will likely always start on the primary as before.