If you put separator widget after 2 widgets (text, ads) then you will get 2 boxes at the top side by side.
If you put separator between them nothing will happen.
Separator widget groups widgets appearing before separator.
So if you put
- 2 widget then separator you will get 2 side by side boxes
- 3 widgets then separator you will get 3 side by side boxes
- 4 widgets then separator you will get 4 side by side boxes
Separator widget will work only if there are at least 2 widgets before it and if it is located on top or bottom widget areas. Top and Bottom widget areas are wide that is why separator will work there. Side widget areas are narrow that is why there is no use of separator there.