Hello! runtimes such as unity have a Skeleton separator https://github.com/pharan/spine-unity-docs/blob/master/spine-unity-skeletonrenderseparator.md that allows one skeleton to be in front and in the back of another.
If one single skeleton should have the animations of both skeletons interacting, I would do as follows:

• in the editor, I'd duplicate the skeleton

• create an animation for each skeleton (maybe using different names for each animation, but it's important that both are selected)

• animate each skeleton interacting

• when the time comes where the two skeletons need to be one in the middle of the other, I might duplicate the "external one" for preview porposes and temporarily hide the slots that should be behind to check out the effect.

• once done with the editor animations, I'd reimport the animation from ne skeleton into the other, and delete all the copies so that only one skeleton remains

• at runtime, I'd use the above link, to insert the second skeleton in the middle