sharing between local "work" and "nightly build" git repos

I'd like to have the following setup: a ~david/git directory, where I
am free to work on things, and a ~david/nightly-git, where a cron job
is going build and test a nightly "next" branch.

I'd like to share as much as possible between the two repos. My naive
first attempt was to clone the local repo (~david/git) using -l and -s
(which I admit I do not completely understand). This sort of worked,
but one issue is that doing a "git pull" in nightly is going to pull
from the *locally*-cloned repo, not from the main git. Another is that
a checkout in nightly failed with the obscure error:

[david@tashtego ~/git-nightly]% git checkout -b nightly-next next
git checkout: updating paths is incompatible with switching branches/forcing
Did you intend to checkout 'next' which can not be resolved as commit?

I assume this is because too much state is being shared the repos, and
something is unfinished in the "git" directory.

I'd love some pointers on how to:

* share as many objects as possible
* share as little state as possible
* make git pull pull from remote in both repos.

Cheers,

- David
-- 
If I have not seen farther, it is because I have stood in the
footsteps of giants.