CloudM Manage has only 1 domain administrator. However, it can have multiple administrators.
CloudM Manage will provide by default an admin role to any user with an admin role in Google Workspace. However, if you wish for a more personalised admin role, then you can create a custom role.
Therefore, to add another administrator to CloudM Manage, you can give a user admin privileges on Google Workspace, or create your own role and assign it to the user.