I first created two repositories, doublepush1 and doublepush2 with default READMEs generated by GitHub.
I then cloned doublepush2 then added a new remote with both URLs
$ git remote add doublepush https://github.com/piannaf/doublepush1.git
$ git remote set-url --add doublepush https://github.com/piannaf/doublepush2.git
When I tried to push a change to to doublepush using
$ git push doublepush
It failed because the histories didn't match.
Since I first added doublepush1 URL to create the doublepush remote, the doublepush1 URL was configured as both a fetch and push. Adding the doublepush2 URL meant making it a push-only url.
$ git remote -v
doublepush https://github.com/piannaf/doublepush1.git (fetch)
doublepush https://github.com/piannaf/doublepush1.git (push)
doublepush https://github.com/piannaf/doublepush2.git (push)
origin https://github.com/piannaf/doublepush2.git (fetch)
origin https://github.com/piannaf/doublepush2.git (push)
I could make this more robust by creating a seperate fetch/push remote for both URLs, but since my origin and double push fetch from different places, I'll leave that as it is.
So to merge the histories, all I needed to do was git pull doublepush
That resulted in a merge conflict since the default READMEs conflicted.
But now they are the same.
And I've confirmed I can push to both simultaniously with
$ git push doublepush
👍