These don't really meet the details you gave but they are the only fics I know where Spike is with another slayer that haven't already been mentioned...
A Rose By Any Other Name by Kari Mouke
SUMMARY: Buffy dives off the tower and into the body of a newborn girl. She tries to get home for the next sixteen years to no avail. Something keeps her from contacting her loved ones. That is until Drusilla steps in to lend a fang. NC17. Buffy/Spike.
SPOILERS: AU after the Gift
Better chance it is this one - it mucks around with reality/time (hint is in the name right? ha) and I think starts post series then switches back to Sunnydale changing reality with this other slayer involved (I'm not sure it is ages since I read it)
Changing History by chosen
Summary: Can a man without memory find his way back to the woman who claimed to love him? Categories: Time Travel, Crossover (AtS), Season 2, Season 7, Post-Series