Kuating Thank you for sharing your Spine project file.
After checking the actual Spine project, I realized that I had indeed misunderstood the problem. Copying and pasting vertices is useful when you already have two different images whose vertices correspond to the same positions in UV space, and you want to align the vertex positions between them. If that prerequisite is not already met, however, it does not help.
If there were an easy way to reverse the positions of the mesh vertices, you could set up the vertices on one mesh first, reverse the entire vertex layout, and then replace the image path with the other image to complete the setup relatively easily. Unfortunately, Spine does not currently provide a way to do this.
In fact, "rotate/scale vertices in edit mode" is one of the items that has been on our roadmap for quite a long time:
EsotericSoftware/spine-editor60
Flipping normals is a somewhat niche requirement, but being able to scale mesh vertices in Mesh Edit mode would have broader use cases as well, so I hope we can implement this feature sooner rather than later.
In conclusion, unfortunately, with the features currently available, I would say that using backface culling to set up back sides for complex meshes is not really practical when it needs to be done extensively rather than only for a small number of specific setups, such as a cape.
As a side note, one of our example skeletons also includes a coin that rotates to reveal its back side. However, it does not use backface culling. Instead, it switches the displayed attachment at the specific moment when neither face is visible and only the edge can be seen:
https://ja.esotericsoftware.com/spine-examples-coin
If your main goal is not specifically to use backface culling, but simply to create the appearance of the back side becoming visible, this example may be useful as a reference.
