thomasparty_27 Do you mean you would like to add outfits to the skeleton data after importing the skeleton files into Unity? If that is your goal, I think there are several ways to achieve that, but I do not understand why you have two skeletons for it. Do you want to add animations as well as outfits later? Do those two skeletons have the exact same setup or do they have different ones? If their setups are exactly the same, is there any reason why you are not using skins to assign different looks to your skeleton? It is difficult to determine how best to advise you without a little more clarity on your purpose, so could you describe to us what you hope to achieve with that skeleton?

thomasparty_27 Thank you for elaborating your situation!

this project is not mine, i took it from a unity project

Based on this, you cannot use the Spine editor, right? I thought you could use the Spine editor since the attached screenshots show what it looks like when the skeleton is displayed on Spine. If you are able to use Spine, animations can be imported into another skeleton via Import Project in the Spine menu.
You can also import a skeleton into another Spine project via Import Project, and move attachments to another skeleton.

Unfortunately, I have no idea how to easily merge the two skeletons in Unity, maybe Harald can give some good advice for you. However, even if you cannot use the Spine editor, playing an animation of a skeleton on another skeleton in Unity is easy. You can use AnimationReferenceAsset for that purpose, which can be generate by using the Create Animation Reference Assets button in the Preview section for a skeleton data asset: http://esotericsoftware.com/spine-unity#Preview
An example code to set up an animation using an AnimationReferenceAsset can be found here: http://esotericsoftware.com/spine-unity#Setting-Animations
If what you want to achieve is to play the same animation that the skeleton with the skeleton in pic no2 as the skeleton in pic no1, then this might be the solution.

I guess you can generally make the two skeletons look as if they are one skeleton if you overlay the skeletons and play the same animation at the same time, but if you want some parts of the skeleton on the back to appear on the front, you can use the SkeletonRenderSeparator component:
http://esotericsoftware.com/spine-unity#SkeletonRenderSeparator
This allows you to insert a Spine skeleton in the draw order of another Spine skeleton. Please refer to the documentation for detailed usage.

I think that overlaying two skeletons and playing back the same animation at the same time would be wasteful in terms of performance and might not be the best solution, but I hope this will be helpful.