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