If you are trying to add a second Site Collection Admin using Powershell, this sentence will help you to achieve it.

1. First you need to establish a SPO connection using global admin credentials.

2. Use the following sentence filling the brackets with your own information, you can change $true if you want to add the permission or $false if you are going to remove it.

Set-SPOUser -Site https://<Tenant Name>-my.sharepoint.com/personal/<user site>  -LoginName <User> -IsSiteCollectionAdmin $true