Wasn't the original point to be able to push to the second copy when the first is down .. what's the point (other than backup) of a second "working" copy that you can't use.
A repo admin or script could enable pushing permissions to the mirror while the primary is down. The when the primary is back up, fast-forward it and change back. Or, just allowing pulling from it and wait to merge until the primary is fixed.
Alternatively you decide the one of the repositories is the primary one, set up a remote called `mirror` and set-up a `post-receive` hook to:
Now just ensure no one pushes into the mirror directly. Of course this only works if you control the primary repository.