git-duet offers similar functionality to this, but takes advantage of existing git fields so tools like github correctly display both authors.
Not a bad idea. I wonder how this would work with a signed commits.
The raw form of a signed commit is just:
author Daniel Heath <email@example.com> 1460596886 +1000
committer Daniel Heath <firstname.lastname@example.org> 1460596886 +1000
gpgsig -----BEGIN PGP SIGNATURE-----
Version: GnuPG v1
-----END PGP SIGNATURE-----
I see no reason you couldn’t have a second gpgsig block to dual-sign the commit.
I doubt any of the tools know how to read it though, and you would presumably prefer not to have both keys on the same computer.
It would probably work better to have each contributor sign & push a tag.