I'm sorry to say that multiple blend modes are not supported on SkeletonGraphic. The problem is the single submesh limitation of Unity's CanvasRenderer which also prevents support for multiple atlas page textures at a single SkeletonGraphic. So even if you would write your own copy of the Spine/SkeletonGraphic shader for different blend modes, it will not be possible to assign multiple materials at a single CanvasRenderer.
There is an issue ticket that deals with support of blend modes at SkeletonGraphic via a different workaround, but I fear it will take some time until we get to implement this feature:
[unity] SkeletonGraphic blend modes support ยท #1316
Having read your headling again, I just noticed that you might not need multiple materials at the same skeleton.
If screen blend mode is sufficient, then you can simply use this shader I quickly made for you (see attachment). You just have to create a copy of the SkeletonGraphicDefault material and assign the shader.
