Pushing to more than one remote in a single git push command

If $REMOTE_URL is the location to which you want to push to (replace $REMOTE_URL with an actual URL), then, for an existing remote—as indicated by $REMOTE_NAME—you would invoke the following command:

git remote set-url --add --push $REMOTE_NAME $REMOTE_URL

For me, I usually set $REMOTE_NAME to just origin. But of course, for various reasons, you can choose an entirely different remote name.