Shell mode does not slice horizontal surfaces. To double wall thickness at a certain height, not right from the heating bed, the second wall has to start at an angle.
The same is valid if designing a fold at the end of an object to plug in the next segment.
The example you find in the attachment is designed to be printed with 0,6mm wall thickness, COPA Polymide. The second wall is drawn to start at a distance of 0,6mm, i.e. touching the outer wall, gradually reaching 1mm distance. So the Zortrax slicer can decide when its time for the first round offset, and it works fine. The distance of the second, inner wall is 1mm, if using stiffer materials this might be too close and deform the outer wall. However it shows the principle.
Also keep in mind that the surfaces of the part of the fold that exceeds the outer wall reverses its orientation, meaning the inside becomes the outside. So it has to be drawn with an offset of 0,4mm. Slice with "ABS settings", since this 0,4mm might not be correct for other materials, depends on the standard thickness the Zortrax slicer uses for different materials.
Red is the doubled wall, green is the offset fold