I read this story years ago, and I'm not positive but I think it's Post-Chosen. It is Spuffy, but for some reason Buffy & Spike are on separate teams travelling the world to trouble spots. Xander is with Spike acting as kind of his Watcher and I think for part of the story they are in the Deep South, then later on there's a big reunion with Buffy and her team. I have no clue about title or author, but I do remember it being very well written.