Trouble in use
Marking pens ：Why the thin tip of dual-pen gets blurred
For a batting twin, a thin core may not be able to be written first, regardless of which nib you are using. For example, you may not be able to write a narrow core when using only a thick core.
These can be seen in the twin of the fiber core, but especially in the case of the twin of the fiber 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 because the plastic core is weaker than the fiber core, and because the capillary power is weak, the ink in the batting is held, and the force to induce is weak.