The recommendation is for one and only one Administrator per blog. Warning: Please be very careful when adding a new Administrator user to your blog. Other Administrators are effectively co-owners. They have as much power over your blog as you do, including the ability to permanently delete it.
For more details please read: user roles adding users changing user roles
Please note that you need to transfer the blog to another user if you want to change the blog owner’s user role to something other than an Administrator.