You can add up to five email accounts in Yahoo Mail.

  1. Mouse over your Yahoo ID.
  2. Click Add or manage accounts.
    - The 'Manage accounts' window opens.
  3. Click Add account.
  4. Enter the account's email address and password, then click Sign in.
When you mouse over the Yahoo ID in your mail, you'll see the new account added. All you have to do is the click the other account to return to it.


More info visit

https://help.yahoo.com/kb/SLN3527.html