Actually, two nibs, four tines, two feeds offer a huge field for experiments, and the final performance depends on the configuration of all those elements. Beyond my first approach –asymmetric vs. symmetric nib sets— there is room for a more careful analysis.
A basic feature of these nibs is their capability to change the line width. In some cases, in a very drastic way —either a fine line or a wide-as-a-brush line—; in some other, the change can be more progressive and it depends mostly on the angle between pen and paper. But there is a lot more.
The different curvatures in the nib sets allow for different line widths. On the Senator, top pen, the change between a thin and a very thick lines is more progressive than on the Pelikan on the bottom.
Asymmetric sets, with one of the nibs basically untouched, preserve the possible flexibility of the upper nib. Then, the effect of pressing the pen on the paper is more complex: lifting the upper tip uncovers that of the second, lower, nib, and this one provides its own supply of ink. So, the thicker line does not rely on just one feed but on both of them, and the whole set is less prone to ink starvation.
On more symmetric structures, with both nibs bended, the whole set becomes fairly rigid. This was in fact the purpose of one of Mr. Yamada’s creations—a fountain pen for a user who tended to push the pen down a lot when writing. In a sense, this idea is not new, and nibs under the name of script or manifold or posting were successful to provide a rigid tool to be used with carbon paper.
On these symmetric sets, the final configuration of the nib points can also favor the use of the pen at non-conventional angles. Or, at least, with the nib rotated about 90 degrees with respect to the natural position. At this angle there are two half-spheres of iridium, belonging to two different nibs, and both with ink ready. At this writing angle, the set is very rigid. This characteristic, though, is not limited to symmetric nibs and asymmetric units can also write at non-conventional angles.
Symmetric sets allow for writing with the tip, of course, and with both slits. The later lays fairly broad lines.
Conclusion: In the aim of creating nibs with several line widths, the Mr. Yamada’s idea of opposing two nibs offers new variables and degrees of freedom. The whole nib has now two feeds and two slits, and each nib-feed set works very independently from the other until the point in which both tips come together.
My thanks to Mrs. Arai and to Mr. Yamada.