Sometimes, it's possible to be logged into one Yahoo! account inside the app, such that it's not possible from the app itself to add the second Yahoo! account.
Don't worry though, as this is easy to setup from a web browser, on any device!
- Head to Yahoo.com in your browser and make sure you are logged in to the account you wish to add
- Go to https://my.footballguys.com
- Go to Edit My Leagues
- Hit the "Add Leagues" button
- Select Yahoo and proceed as before with the first league
- When you next open the app, the second Yahoo account will show with its leagues