Hmm... That's a good question and a practical use.
I'll get back to you on it later. I need to check the APIs.
But, in theory, it should be as simple as copying the Skeleton object from one and assigning it to the other.
I'll also be adding the solution to the Mix and Match sample scenes for future reference.
This feature requires a minor change to the SkeletonGraphic component so expect it a little bit later. To be included in the next update.
The git repo and the unitypackages have been updated with a new component named "SkeletonGraphicMirror".
You add that component to your SkeletonGraphic GameObject.
Then set your SkeletonAnimation as the source. See the inspector for more options.
If you are using MixAndMatch, and especially if you are repacking, you need to call SkeletonGraphicMirror.UpdateTexture(repackedTexture)
to apply it correctly. Otherwise, you'll get a jumbled up looking skeleton.
SkeletonGraphicMirror is a small component. You can read the code here: spine-runtimes/SkeletonGraphicMirror.cs at 3.6