Skin of the 2nd player cannot be changed.

I solved this problem.

1. Upload Minecraft save data to Nintendo cloud backup.

2. Remov all Mincraft game data both save and soft.

3. Re-install Minecraft soft and download saved data from cloud.

4. Logout Xbox account and clear login data.

5. Re-login Xbox account and restart Minecraft.

Then you can change skin of 2nd player's.

Thank you for your report!
We're tracking this issue in MCPE-135132, so this ticket is being resolved and linked as a duplicate.

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