On the screenshot, there are actually two heads in the same coordinate. In the sculpting mode, the selected head (or head 1) is in the yellow color, whereas the unselected head (or head 2) is in the darker-yellow color. Apparently, these two heads have different facial features.
Now, how do I combine these two heads to form a new head?
I) The new head must have the eye sockets, the nose and the ears from the head 1, and have the cheek and mouth from the head 2.
II) And the connection between the meshes should be seamless.
I have tried to use the FaceSet brush from Blender 2.93, and it seems not convenient to use. For example, after using it to define a new FaceSet which contains the eye sockets and nose from head 1, I don't know how to replicate the exact area of this new FaceSet to contain the eye sockets and nose from head 2, so that I can delete the exact area on the head 2. If the replicated area is not exact, then the two meshes are not matching, and this causes a giant seam on the new head after using the Boolean modifier to union the two meshes. Removing any big seam could actually alter and ruin the mesh of the new head.
Furthermore, even if the two meshes are almost exact matched, I still do not know the most non-invasive method to make the connection between the meshes seamless. If the method is invasive, like an invasive surgery, then the beautiful face is ruined by the surgery knife. Thank you for reading!