If your Yahoo! Business Email and Yahoo! Mail addresses are associated with the same Yahoo! ID, you can manage them both from the Yahoo! Mail interface, and easily switch between accounts.
Note: While you can switch between your Business Mail and associated free Yahoo! addresses anytime, you cannot switch between email accounts linked to different Yahoo! IDs. If you have accounts associated with different IDs, you'll have to sign out of the first account, then sign in to the second using your other Yahoo! ID and password.