You should add another remote for my-app-prod named prod-heroku (replace GIT_URL with the Git URL that you can find in the settings page of my-app-prod in heroku):
git remote add prod-heroku GIT_URL
git push prod-heroku production:master
This will push your local branch production to the remote branch master in prod-heroku so my-app-prod will get deployed with the code in production branch.