Sorry for the delayed response.
Typically a Spine skeleton is used per animated entity in your game. Eg, you'd have a single skeleton for an enemy and everywhere it appeared you'd use an instance of that skeleton. Note you can have multiple skeletons in a single Spine project.
You are using Spine as a level editor? If you have the same entity on multiple levels and you make that part of a massive level skeleton, you'd have to keep the animations for that entity in sync across multiple level skeletons. What happens when you have many levels? This wouldn't be easily manageable, even if Spine did allow you to copy between skeletons. Unfortunately it currently doesn't allow that. The issue is here:
https://waffle.io/EsotericSoftware/spine/cards/580bacfc29fef328007f9dc5
