Make sure that your .git/config has the following settings:
[remote "origin"]
fetch = +refs/heads/*:refs/remotes/origin/*
url = git+ssh://git@git.overlays.gentoo.org/proj/python.git
pushurl = git@github.com:gentoo/python.git
pushurl = git+ssh://git@git.overlays.gentoo.org/proj/python.git
[remote "github"]
url = git@github.com:gentoo/python.git
fetch = +refs/heads/*:refs/remotes/github/*
You can pull/push to both repos at once using --all switch.