>>1018731
A second layer can be useful for adding the illusion of depth, but it doesn't wholly solve your problem. If you want to avoid the underside of the skull being bare from certain angles, you have two options.

One option is to duplicate a portion of the head mesh, size it up slightly, and color it the same as the hair. That'll create a skullcap for your model. You can extrude or move the outermost fringe of verts into the model to avoid a "dead zone" between the skin mesh and the skullcap mesh.

Another is to extrude a portion of that inner curtain and "tuck it" under the skull and into the base of the neck. See the attached image. That creates the illusion of the short hairs at the bottom of the neck. It looks weird in isolation but it works in practice.

Given your model's short haircut, I'd say a short inside curtain + a skullcap is probably your best best. The "tuck in" approach seems like it'd be more obviously fake with a short-haired model, since you could see more of it. But you could give it a try if the skullcap fails.

You could also directly recolor portions of the head mesh for the skullcap look, but I'm not sure that's viable in your case, looking at your model and the size of the quads. Resizing a duplicated portion of the skull mesh seems more suitable.