Two accounts, one personal, one business. I want separate logon & passwords, as the business account is shared.

Just got ATT fiber internet at my business location.  I've had ATT UVerse at my home for several years.  Setup the business account with a different email address, and (I thought) a different password.  ATT now only recognizes one password for both accounts regardless of userID.  I have tried logging on with either userID, but get an "incorrect password" message, and if I change the password for one, ATT/Yahoo changes it for the other also.  I don't want anyone else in my personal account, but I need to share the business account with someone else (bookkeeper), so that service and billing emails can be accessed by both.  How can I separate the two accounts so that when the bookkeeper logs in to the business account, he can use a separate password and not have access to my personal account?  This is maddening.