I tried to use setSkinByName twice and it just replaced the previous skin instead of displaying two (
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I found the answer:
let newSkin = new spine.Skin("new-skin");
newSkin.addSkin(player.skeleton.data.findSkin("body"));
newSkin.addSkin(player.skeleton.data.findSkin("ears"));
...
newSkin.addSkin(player.skeleton.data.findSkin("body"));
newSkin.addSkin(player.skeleton.data.findSkin("ears"));
...