When this option is checked and the launcher is configured to rotate according to the orientation, then it will be possible to assign one position for an item in portrait mode, and another position in landscape mode.
The portrait position is the primary one. It means that without special configuration, an item will use the same position in landscape as the position in portrait. This is the default behavior, but as soon as you move an item in landscape (when this dual position option is checked), then Lightning Launcher will remember it and dissociate portrait/landscape settings. Move back the orientation to portrait and the item will move back to its original place. Rotate to landscape again, and the item will get the position for the landscape mode.