Sounds like
Crossing Over into Unchipped Country and
Crossing Shadow River by Dutchbuffy2305
Crossing Into Unchipped Country - by Dutchbuffy2305 [PG-13]
Summary: Buffy gets sucked into an alternate reality where she meets another Spike. Hijinks ensue.
Crossing Shadow River - by Dutchbuffy2305 [NC-17]
Summary: About ten years after season 5 of AtS; sequel of sorts to Crossing into Unchipped Territory. You should read that if you want to know everything about how to cross dimensions, and how Buffy and Spike got to be together. If you insist in reading this first, this is the recap: Buffy met evil Spike when she accidentally fell in another dimension. Hijinks ensue. He returns souled Spike to her (I'm not telling how) and they lived happily ever after. I still think you should go read it first, because my summary doesn't quite do it justice...