In recent versions of Git you can add multiple pushurls for a given remote. Use the following to add two pushurls to your origin:
git remote set-url --add --push origin git://original/repo.git
git remote set-url --add --push origin git://another/repo.git
So when you push to origin, it will push to both repositories.
UPDATE 1: Git 1.8.0.1 and 1.8.1 (and possibly other versions) seem to have a bug that causes --add to replace the original URL the first time you use it, so you need to re-add the original URL using the same command. Doing git remote -v should reveal the current URLs for each remote.
UPDATE 2: Junio C. Hamano, the Git maintainer, explained it's how it was designed. Doing git remote set-url --add --push <remote_name> <url> adds a pushurl for a given remote, which overrides the default URL for pushes. However, you may add multiple pushurls for a given remote, which then allows you to push to multiple remotes using a single git push. You can verify this behavior below:
$ git clone git://original/repo.git
$ git remote -v
origin git://original/repo.git (fetch)
origin git://original/repo.git (push)
$ git config -l | grep '^remote.'
remote.origin.url=git://original/repo.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
Now, if you want to push to two or more repositories using a single command, you may create a new remote named all (as suggested by @Adam Nelson in comments), or keep using the origin, though the latter name is less descriptive for this purpose. If you still want to use origin, skip the following step, and use origin instead of all in all other steps.
So let's add a new remote called all that we'll reference later when pushing to multiple repositories:
$ git remote add all git://original/repo.git
$ git remote -v
all git://original/repo.git (fetch) <-- ADDED
all git://original/repo.git (push) <-- ADDED
origin git://original/repo.git (fetch)
origin git://original/repo.git (push)
$ git config -l | grep '^remote.all'
remote.all.url=git://original/repo.git <-- ADDED
remote.all.fetch=+refs/heads/*:refs/remotes/all/* <-- ADDED
Then let's add a pushurl to the all remote, pointing to another repository:
$ git remote set-url --add --push all git://another/repo.git
$ git remote -v
all git://original/repo.git (fetch)
all git://another/repo.git (push) <-- CHANGED
origin git://original/repo.git (fetch)
origin git://original/repo.git (push)
$ git config -l | grep '^remote.all'
remote.all.url=git://original/repo.git
remote.all.fetch=+refs/heads/*:refs/remotes/all/*
remote.all.pushurl=git://another/repo.git <-- ADDED
Here git remote -v shows the new pushurl for push, so if you do git push all master, it will push the master branch to git://another/repo.git only. This shows how pushurl overrides the default url (remote.all.url).
Now let's add another pushurl pointing to the original repository:
$ git remote set-url --add --push all git://original/repo.git
$ git remote -v
all git://original/repo.git (fetch)
all git://another/repo.git (push)
all git://original/repo.git (push) <-- ADDED
origin git://original/repo.git (fetch)
origin git://original/repo.git (push)
$ git config -l | grep '^remote.all'
remote.all.url=git://original/repo.git
remote.all.fetch=+refs/heads/*:refs/remotes/all/*
remote.all.pushurl=git://another/repo.git
remote.all.pushurl=git://original/repo.git <-- ADDED
You see both pushurls we added are kept. Now a single git push all master will push the master branch to both git://another/repo.git and git://original/repo.git.
IMPORTANT NOTE: If your remotes have distinct rules (hooks) to accept/reject a push, one remote may accept it while the other doesn't. Therefore, if you want them to have the exact same history, you'll need to fix your commits locally to make them acceptable by both remotes and push again, or you might end up in a situation where you can only fix it by rewriting history (using push -f), and that could cause problems for people that have already pulled your previous changes from the repo.
