A repository is on a GitHub account and there is already another Github account linked to Heroku. Can I link multiple GitHub accounts to my Heroku account so that one application can access the repository and access another repository with another Github account?
Using multiple GitHub accounts with one Heroku account isn't possible. I would recommend giving access to the target repository to the same GitHub account. If that is not an option, then it is also possible that you could have two Heroku accounts, each which map to a specific GitHub account.