To copy an animation from one project to another, especially when the skeletons are largely the same but have some differences (like your angel and devil skeletons), you can use the Import Project feature in Spine. This allows you to import animations from one skeleton to another, provided the necessary bones, slots, and attachments exist in both skeletons. Since the devil doesn't have wings, the animation related to the wings won't affect the devil skeleton, as those specific bones don't exist in the devil's skeleton.

Here's a step-by-step guide on how to do it:

1. Open the project where you want to import the animation (in this case, the devil's project).
2. Click on the Spine logo to open the main menu and select Import Project.
3. Choose the project file of the angel (the source of the animation).
4. Select the animation you want to import and proceed with the import.

This process will import the walking animation from the angel to the devil, ignoring the wing movements since the devil skeleton does not have the wing bones. Make sure that all other bone names, slots, and attachments that are animated and exist in both skeletons have the same names for a successful import.

Spinebot gave good info, use Import Project.

Note you can have multiple skeletons in the same project. That can save you some jumping back and forth between projects.

In general copying your animations isn't great, as now you have to keep them in sync if they change. Same with having 2 skeletons that are similar, if you change one you may need to change the other. What happens when you have 3-4 skeletons or animations? Or a dozen, or a hundred?

You could use skin bones and constraints to combine multiple characters into a single skeleton. For example, an angle skin could have wing bones. This scales better to having many characters, or characters that are highly customizable.

That said, if you truly have different skeletons that don't share bone setup or animations, then using a separate skeleton makes sense.