Hey Crocodilium! that part of the changelog just means that Spine is not badly crashing if you attempt to do it, but it's not fully supported yet I think.

On the topic, it might be interesting for you to read this topic: Changing poses within a single animation

Pointing it out because I was copying keyframes from a skeleton to another one, but one frame at a time and not all at once. It worked, so maybe you'll find it useful as well.

Erikari escreveu

1. Export all the poses in a json from photoshop and import them into your project under a bone called "alt poses" that you will later delete, just to keep things tidy

2. Import your spine project inside your spine project as a second skeleton and call that skeleton "backup", then deactivate the backup skeleton
3. Isolate all the alt images for a pose and make them visible. Then start aligning the skeleton to the images.
4. put each alt image in its righful slot, mesh and bind the images that need it
5. import the project again and name the skeleton with the alt pose you rigged, deactivate it
6. activate your backup skeleton, select all the bones (I generally store the selection with ctrl+1 so that I can recall it fast by pressing 1) then in world mode press ctrl+c
7. activate your original skeleton, select all the bones (it's easier to do if you filter the tree for bones, then select all of them, IKs too.) and press ctrl+v.
   it's important to have the IK all with the same bending, so if you've changed it, take it back to restore the pose or ugly things will happen.
   it's also important to have the same number of bones and to not having deactivated "inherit rotation" etc. for the sake of this part, you can reactivate them when you've finished though.
8. repeat until there's no alt images left.
9. take your original skeleton, if some meshes turned ugly in setup pose (it can happen because of a rotation bug we currently are in the process of fixing) simply copy them from the backup skeleton and move them in place of the distorted meshes.
10. in animate mode create animations for the alt poses. Do so by selecting the bones of the alt skeletons that were prepared before, copy them and paste them inside the animation.
11. you can delete the alt poses skeletons.

Also keep in mind that you can always import the animations from one skeleton to the other and if I were you I'd definitely do that for multiple keys.

To confirm, currently keys can't be copied to another skeleton. If you try, when you paste, the keys will be pasted to the skeleton they were copied from.

I was looking for this! I thought I was so smart! I transform constrained my IK controls to an FK skeleton, and set world transform keys from it, hoping to copy them over

Hmm, maybe you could do that by moving the FK bones into the same skeleton.

This is so irritating when you switch from AE to Spine. i can not copy and paste animation to different skeleton ? any one can pls help.. spine seems so much empty. or may be i don't know things

Hello! This is an year old post.

You can directly import an animation from a skeleton to another, as long as the structure and the names of the bones match.

You still cannot copy keyframes from one skeleton to another, but you can copy bone transforms, meaning that you can select a bone, press ctrl+c then select a bone in a different skeleton and paste all the transforms for the bone at the keyframe you copied.

What is it that you would like to achieve exactly? And why are you copying keyframes from a skeleton to another (to have a bit more of context)?