If the second user is simply going to access the repository (and not write to it) you could always just serve it over HTTP or HTTPS and use basic authentication to protect it. That way you don't need to worry about users and permissions and what-not.
Just create a bare repository on your slice and set some permissions for the web server:
mkdir example-repo.git
cd !$
git --bare init --shared=group
git --bare update-server-info
sudo chown -R exampleuser:www-data example-repo.git
sudo chmod -R ug+r example-repo.git
And in Apache, protect the repository with something like:
<Location /example-repo.git>
AuthType Basic
AuthName "Example Repository"
AuthUserFile /home/example/.htpasswd
Require valid-user
</Location>