FFD keys are per attachment, the reason being that each attachment has its own mesh. The meshes may be different, either with a different number of vertices or just in a different order. Ie, when you move a mesh vertex, there is no way to know what vertex you want moved in a mesh for a different attachment. It only makes sense if you duplicate a mesh and then change the image, because then you have exactly the same mesh.
There are two use cases:
1) You have two attachments for the same slot and want to use FFD to deform them differently. Spine handles this, since FFD keys are specific to a mesh attachment.
2) You have two attachments for the same slot and want to use FFD to deform them the same (eg the flags in your video). Spine currently doesn't have a way to reuse the FFD keys for a different mesh. ๐ I'm open to discussing how this might work.
The simplest is probably copy/pasting FFD keys, similar to how copy/pasting bone keys works (copy FFD keys, select different mesh, paste and it does the best it can to match the deformation). This may save you the work of animating the same mesh repeatedly, but is unfortunate in that the data is duplicated. Eg, if you change your flag deform, you'll have to change it for every flag mesh.
There may be some way of associating multiple meshes with the same FFD keys. This has some complexity with the UI and making sure the user understands what is going on.
Another possibility might be having only a single mesh attachment and the FFD keys are only for that mesh, as they are now, but somehow allowing the image used for the mesh to change. Conceptually this is different from keying a slot attachment change. It would have to show up as a separate row for the mesh in the dopesheet, similar to how FFD keys are for a specific mesh. The UVs are normalized, so nothing will break if an image is a different size, it will just be stretched.
There is the question of how to choose the image to show. It could be that in setup mode you can drag any number of images under a mesh, then you can show any one of those in animate mode. I'm not sure where to put the key button though, as it would make sense to put it next to the mesh, just like a slot attachment change, but this is already used for keying the mesh's FFD.
Skinning works by bones manipulating mesh vertices. The bones are outside the mesh, so unlike a mesh's vertices, the bones stay constant when a different mesh is shown. The deform of a mesh from skinning depends on the weights set for the mesh's vertices. If you have many meshes, you will need to set the weights for each mesh, which is how much each bone affects each mesh vertex. This is likely much easier than duplicating your flag deform, whether by hand or via copy/paste, but would still be quite tedious if you have many flags. Also, we want FFD to be usable on top of skinning to correct for situations where skinning performs poorly (eg extreme bends). Any FFD corrections would still have to be done for each mesh. It would seem a solution for reusing a mesh with different images is useful even when using skinning.
This could be worked around at runtime by simply changing the texture region a mesh attachment uses. This is not quite as nice as being able to key it in the editor, but it could work well for a situation like the flags and is something you can do until we concoct a more robust solution.