Thank you for your answer @Erika !

What should I do when those common parts should disappear?

For example, this is the "defeat" skin (the bones) selected, but all the slots with multiple attachments not included in any skin placeholder are also shown:

How could I avoid these parts showing up if they are not linked to a placeholder (as they should change, for example, opening and closing hands)?

Hello again, @Erika, I've been working on this project and found a simple solution that works for me.

The issue now I have is about mixing skins.

I want to achieve what happens when I pin 2 skins in my preview window:

Each one has it own placeholder:

(this is the same for every part)

And I'm trying to do what the mixing skins documentation mentions here:
https://es.esotericsoftware.com/spine-unity-mix-and-match

However, instead of achieving this:

I'm getting this:

The code where I try to mix both skins is

private const string shockingWithBones = "shockWithBone";
[SerializeField] [SpineSkin] private string bonesSkin;
[SerializeField] [SpineSkin] private string shockSkin;
[...]
mixSkin = new Skin(shockingWithBones);
mixSkin.AddSkin(skeletonData.FindSkin(shockSkin));
mixSkin.AddSkin(skeletonData.FindSkin(bonesSkin));
skeleton.SetSkin(mixSkin);

I also tried changing the addskin lines order, with no change.

Could you help me with this?
Thank you very much in advance, let me know if you need anything else to explain the problem!

Nate The order you add the skins matters.

As I said, it is not working either, at last, the visible result is the same

The skin change is responding a spine event, here the complete script logic related to this:

public void Shocking()
    {
        TrackEntry shockAnimation = spineAnimationState.SetAnimation(0, shock, false);
        shockAnimation.Event += EventManager;
        spineAnimationState.SetAnimation(4, hideParts, true);
        shockAnimation.Complete += EndShock;
    }
    void EndShock(TrackEntry track)
    {
        spineAnimationState.ClearTrack(4);
        skeleton.SetSkin(normalSkin);
        currentSkin = normalSkin;
        skeleton.SetSlotsToSetupPose();
        spineAnimationState.AddAnimation(0, idleBody, true, 0);
    }
void EventManager(TrackEntry trackEntry, Spine.Event e)
    {

        switch (e.Data.Name)
        {
         
            case "BonesSwitch":
                if (bonesOn)
                {
                    skeleton.SetSkin(shockSkin);
                    currentSkin = shockSkin;
                    bonesOn = false;
                    break;
                }
                else
                {
                    mixSkin = new Skin(shockingWithBones);
                    mixSkin.AddSkin(skeletonData.FindSkin(bonesSkin));
                    mixSkin.AddSkin(skeletonData.FindSkin(shockSkin));
                    skeleton.SetSkin(mixSkin);
                    bonesOn = true;
                    break;
                }

            default:
                break;
        }
    }

Thank you!

Alright, brief update:

changing "mixSkin.AddSkin(skeletonData.FindSkin(bonesSkin));" with the actual name of the skin ("Shock-Bones") made this work.

However, how could I reference the skin as defined in the inspector?

Sorry, sometimes I have troubles explaining myself as my language is not English.

I meant I created a variable ([SerializeField] [SpineSkin] private string shockSkin and wanted to use "shockSkin" instead of the string name, and... I realised this is possible as long the problem is not that, it was the lack of SetSlotsToSetupPose() line. I'm sorry.

Skins is a really difficult topic to me to understand, so I apologize for my silly questions. However, with your guidance and my brute force testing, I found a way to achieve what I wanted, so I'm pretty thankful.

I'm talking with my game's producer to include you all in the game credits once it is launched haha!
Thank you all again!