Knowledge of marking pens

The ink of one of the twin tip disappears first.

For a batting twin, a thin core may not be able to be written first, regardless of which nib you are using.
This is because the capillary power of the two nib is different, the ink in the batting is kept, and the difference is derived by the force to induce it.
These can be seen in the twin of the Extra-fine tip , but especially in the case of the twin of the Extra-fine tip core and the plastic core, the plastic core might not be able to be written ahead from the difference in the form of each pen tip.
This is due to the weak capillary power of the plastic core compared to the Extra-fine tip , and the power to induce the ink in the batting.