Getting an error pushing to github - Updates were rejected because a pushed branch tip is behind its remote

If you don't care about what's currently in repo2 and are confident that totally overwriting it is ok then you can use:

$ git push -f [email protected]:<heroku repo name>.git

Remember though that push -f can wipe out other developers changes if they were posted since you last pulled from the repo... so always use with extreme caution on multi-developer teams!
In this case heroku is always downstream and github is where the code is managed and maintained so this makes push -f on heroku a safer option that it would otherwise be.

I am always a big fan of using git pull --rebase and then git push origin master . A couple of the places I have worked since a lot of places do not allow a push -f (especially places that use bitbucket).

git pull --rebase 
git push origin master

The rebase will apply your changes after those already to the remote (online website). This video literally goes over your exact issue and solves it using git pull --rebase https://youtu.be/IhkvMPE9Jxs?t=10m36s

If you pull the other repo first:

git pull repo2

This will merge in the other repos's changes which you can add and commit.

Then you can push the repo back.