Hello again, it looked to me in the video that the vertices simply had the same weights in some cases, such as at 0:24 where some weights are set on the parent bone, while others remain on the child bone even though they're on the edge of said connection, so they aren't really glued together or this effect wouldn't be possible.
If you want to achieve this effect it is already possible in Spine, I use it a lot myself, the two meshes appear connected as long as they share the same weights.
The trick lies in placing vertices in the same points for both meshes (like in the video) and to facilitate this, you can activate the "wireframe" property on one of the mesh to copy the same vertices on the other while in edit mode.
Would you be able to explain in greater detail how this differs compared to what is already possible in Spine? It doesn't look like you can control one vertex instead of two overlapping ones from the video but maybe that is the case?