Hello. Achieving the desired effect is not trivial unfortunately, currently we have no shader or setup included which provides this out of the box.
I have recently replied to this similar thread:
Unity3D 에서 불투명한 스파인 리소스가 겹치는 문제
If you want to make this work, you have to ensure the following:
1) Draw order sorting inside the mesh needs to be changed to front-to-back (Spine uses back-to-front order to handle transparency correctly)
2) ZWrite needs to be enabled
3) ZTest needs to be enabled
4) Alpha Test needs to discard very agressively, everything below 1.0 opacity should be clipped. Otherwise you would receive strange border outline effects.
So currently this would require changing the draw order of a skeleton (code adaptation) and a special shader.
We do not recommend that you perform these steps yourself, since they are not simple.
We will potentially come up with a built-in feature that will provide the functionality in the future, however currently we are busy with some high-priority issues.
Could you let us know your use case? Then we might be able to suggest a suitable solution.