You have set the wrong screen size in the rendering.
When the dialogue for the output size opens, you can enter numbers for width and height. This is NOT the output size for the individual videos (e.g. 1920x1080 for FullHD), but rather the size of the whole screen with the high resolution that you want to split.
To fill in the correct values, select the desired screen in the dropdown and click the button with the dotted outline.
For more details, see also "Multidisplay presentation using split videos" in the WINGS help.