....

One that comes to mind is Holly's Dancing in the Moonlight: http://paragon.dark-solace.org/stories/ditm.htm

Dancing in the Moonlight is the only one I know where they are both slayers.

This is a bit of a sideways step...

Slayer of slayers by jypzrose - as I remember Spike is a vamp but is used to slay slayers and Buffy has been vamped but she retains her soul. Totally rewrites Spike's history. As I said, a sideways step but produces similar dynamics.

(a bit of a spoiler... as I remember it Spike's origin is as a watcher)

Hello, Cutie by scrambled_thoughts is another one: http://spikeluver.com/SpuffyRealm/viewstory.php?sid=32857&warning=NC-17

I have a couple - in Life Goes On, Spike has shashued after NFA and turns out to be a slayer. It ends with that discovery though. It doesn't go on with his life as such.

Things That Go Bump in the Night - again, Spike turns out to be a slayer. All of the following shorter Fics (A Very Bumpy Christmas, etc) involved slayer Spike and slayer Buffy - right up until the end of their lives. Most of them were written for Schmoop Bingo prompts and are quite short.

Those that aren't on here, can be found at http://spuffystuff.org/Fics.html