I'm not sure if this story was ever finished, or if I just lost track of it before then... but it was Spike/Buffy and Spike/Tara (or, possibly more accurately, William/Tara), though at the point I'd last read, the Spike/Tara aspects were not very apparent... they were friends who might become something more.
Anyway, the basic plot was this: Buffy dies (I think this was a post-"The Gift" fic), Willow is pretty much insane from magic use, the remaining Scoobies set up a legal family structure to protect Dawn and themselves, and Spike has gotten his soul. Buffy comes back and Spike is still in love with her, but after getting the soul, that part of him has fallen in love with Tara.
I remember a scene that I'm pretty sure is from the same fic where Tara goes to visit Willow wherever she's being held. Willow is definitely not very sane and taunts Tara about her relationship with Spike.
Is this ringing any bells?
ETA:
alshder had the answer... it was "The Returning" by
theohara.