I am using Display Fusion to place separate wallpapers on two monitors.
But when I do so neither fills the screen.
Both seem to be in the TILE setting.
When I go to windows display properties and reset it to STRETCH I then get two very thin, very tall wallpapers on each screen.

How can I fix this?

You have to change it in DF itself. Right below where you set the wallpaper is the option.