When configuring the display positions, the user can freely move it and when two screens touch each other eg. on the bottom and top edge, they can be moved horizontally against each other. However, there's no additional help from the SystemSettings to center one screen relative to the other, besides some position numbers no one reads. It'd be convenient if the relative center is set, it'd be a little bit harder to move it or a visual hint like, some indicator lines, would be displayed.
*** Bug 461647 has been marked as a duplicate of this bug. ***
Tried fixing it, but it turned out not as straightforward as copying vertical snapping function and replacing y/height with x/width. Will look into it sometime later.