If you want to merge your At&t and yahoo and you don't know how to merge At&t and yahoo. merge people got to use a merged account combined account whenever they can access the service of both At&t and yahoo follow this steps to merge At&t and yahoo
:- you can use same password for both account
:- sign into your At&t account using your yahoo email address
:- you can sign in using your your yahoo as well as At&t mail for both account
:- you are getting emails from both account at the same place
Read more :- https://getassist.net/unmerge-yahoo-mail-from-att/