I have tested this configuration :
I tried with "dual screen mode" checked and unchecked.
As shown in the image, I am not able to extend the size of the window bigger than the height of the first monitor, so impossible to make a single window covering the surface of two monitors
Then the idea to duplicate the display and have a first window cropped on the top part, full screen displayed on first monitor,
and a second window cropped on the below part for the second monitor
There are some layouts that are "flat" that are well fit for this separation, and I will priviledge these ones.