If you have two completely rigid objects, then as soon as they touch on two points, those two points define an axis. Rotate the objects on this axis until they touch at a third point. Now they are fixed relative to each other, and you can't move them to touch at more points. If you slide the objects on each other, as soon as they touch at a "new" point, one of the other three will be slightly further away, and won't touch any more. Unless you use a flexible material, or somehow make a perfectly flat surface, they will only touch at three points.